Data posted to Apple by Spotlight (Bulky JSON payloads have been reformatted for readability). Note the X-Apple-UserGuid -- this appears to remain constant across all requests.

0_completion_post.txt

POST /fb?key=andromeda HTTP/1.1
Host: api.smoot.apple.com
Content-Type: application/json
Accept-Charset: utf-8
Connection: keep-alive
X-Apple-ActionSignature: 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
Accept: application/json
X-Apple-UserGuid: 267af341-df6c-4eed-5e78-a2b8a49a1d1f
User-Agent: (OS X 14A389) Spotlight/916
Content-Length: 5991
Accept-Encoding: gzip, deflate
Accept-Language: en-us
[
  {
    "fbq": "eyJ1IjoiMjY3YWYzNDEtZGY2Yy00ZWVkLTVlNzgtYTJiOGE0OWExZDFmIiwiZSI6ImIiLCJwIjoiY29ubmVjdGlvbiIsInEiOiJjb25uZWN0aW9uIiwidHMiOjE0MTM2MDgwOTksImciOm51bGwsImEiOiJzcG90bGlnaHQiLCJkIjoibWFjIiwibCI6ImVuX1VTIiwiaSI6IjE0MzQ0MS0xLDIxIiwiYyI6bnVsbCwiZWN2IjowLCJjY3YiOjB9",
    "results": [
      "sys_pref",
      "server:eyJkIjoid2ViX3N1Z2dlc3QiLCJyIjoid2I6aHR0cDovL2Nvbm5lY3Rpb25zYWNhZGVteS5jb20ifQ==",
      "dictionary",
      "documents",
      "folders",
      "other",
      "developer",
      "server:eyJkIjoiYmluZyIsInIiOiJiOmh0dHAlM0ElMkYlMkZ3d3cuY29ubmVjdGlvbnNhY2FkZW15LmNvbSUyRmhvbWUuYXNweCJ9",
      "server:eyJkIjoiYmluZyIsInIiOiJiOmh0dHAlM0ElMkYlMkZ3d3cuY29ubmVjdGlvbnMuY29tJTJGIn0=",
      "server:eyJkIjoiYmluZyIsInIiOiJiOmh0dHAlM0ElMkYlMkZjb25uZWN0aW9uc2NsZXZlbGFuZC5vcmclMkYifQ=="
    ],
    "timestamp": 217690,
    "input": "connection",
    "type": "render",
    "trigger_timestamp": 217690
  },
  {
    "latency": 406,
    "input": " ",
    "ttfb": 406,
    "delta_ref": "spotlight_local_search",
    "headers": {
      "User-Agent": "(OS X 14A389) Spotlight\/916",
      "X-Apple-UserGuid": "267af341-df6c-4eed-5e78-a2b8a49a1d1f",
      "Accept-Charset": "utf-8",
      "X-Apple-CachedResults": "[]",
      "Accept-Encoding": "gzip, deflate",
      "X-Apple-CachedQueries": "[]"
    },
    "type": "search_remote",
    "timestamp": 406,
    "delta": -72182,
    "url": "https:\/\/api.smoot.apple.com\/search?q=+&locale=en-US&time_zone=America\/Denver&calendar=gregorian&key=andromeda"
  },
  {
    "delta": 0,
    "timestamp": 1249,
    "input": "S",
    "type": "search_local",
    "ttfb": 0,
    "latency": 83
  },
  {
    "delta": 0,
    "timestamp": 1668,
    "input": "Sp",
    "type": "search_local",
    "ttfb": 0,
    "latency": 52
  },
  {
    "delta": 0,
    "timestamp": 1841,
    "input": "Spo",
    "type": "search_local",
    "ttfb": 0,
    "latency": 50
  },
  {
    "delta": 0,
    "timestamp": 1935,
    "input": "Spot",
    "type": "search_local",
    "ttfb": 0,
    "latency": 41
  },
  {
    "delta": 0,
    "timestamp": 2151,
    "input": "Spotl",
    "type": "search_local",
    "ttfb": 0,
    "latency": 40
  },
  {
    "delta": 0,
    "timestamp": 2354,
    "input": "Spotli",
    "type": "search_local",
    "ttfb": 0,
    "latency": 44
  },
  {
    "delta": 0,
    "timestamp": 2449,
    "input": "Spotlig",
    "type": "search_local",
    "ttfb": 0,
    "latency": 35
  },
  {
    "delta": 0,
    "timestamp": 2548,
    "input": "Spotligh",
    "type": "search_local",
    "ttfb": 0,
    "latency": 36
  },
  {
    "delta": 0,
    "timestamp": 2613,
    "input": "Spotlight",
    "type": "search_local",
    "ttfb": 0,
    "latency": 38
  },
  {
    "delta": 0,
    "timestamp": 2901,
    "input": "Spotlight I",
    "type": "search_local",
    "ttfb": 0,
    "latency": 70
  },
  {
    "delta": 0,
    "timestamp": 3070,
    "input": "Spotlight In",
    "type": "search_local",
    "ttfb": 0,
    "latency": 54
  },
  {
    "delta": 0,
    "timestamp": 3151,
    "input": "Spotlight Inc",
    "type": "search_local",
    "ttfb": 0,
    "latency": 49
  },
  {
    "delta": 0,
    "timestamp": 3235,
    "input": "Spotlight Ince",
    "type": "search_local",
    "ttfb": 0,
    "latency": 53
  },
  {
    "delta": 0,
    "timestamp": 3378,
    "input": "Spotlight Incep",
    "type": "search_local",
    "ttfb": 0,
    "latency": 44
  },
  {
    "delta": 0,
    "timestamp": 3459,
    "input": "Spotlight Incept",
    "type": "search_local",
    "ttfb": 0,
    "latency": 44
  },
  {
    "delta": 0,
    "timestamp": 3571,
    "input": "Spotlight Incepti",
    "type": "search_local",
    "ttfb": 0,
    "latency": 45
  },
  {
    "delta": 0,
    "timestamp": 3642,
    "input": "Spotlight Inceptio",
    "type": "search_local",
    "ttfb": 0,
    "latency": 44
  },
  {
    "delta": 0,
    "timestamp": 3862,
    "input": "Spotlight Inception",
    "type": "search_local",
    "ttfb": 0,
    "latency": 46
  },
  {
    "latency": 344,
    "input": "Spotlight Inception",
    "ttfb": 344,
    "delta_ref": "spotlight_local_search",
    "headers": {
      "Accept-Charset": "utf-8",
      "X-Apple-UI-Scale": "1.000000",
      "X-Apple-CachedResults": "[]",
      "X-Apple-UserGuid": "267af341-df6c-4eed-5e78-a2b8a49a1d1f",
      "User-Agent": "(OS X 14A389) Spotlight\/916",
      "Accept-Encoding": "gzip, deflate",
      "X-Apple-CachedQueries": "[]"
    },
    "type": "search_web",
    "timestamp": 4167,
    "delta": 305,
    "url": "https:\/\/api.smoot.apple.com\/websearch?q=Spotlight+Inception&locale=en-US&time_zone=America\/Denver&calendar=gregorian&key=andromeda"
  },
  {
    "latency": 367,
    "input": "Spotlight Inception",
    "ttfb": 367,
    "delta_ref": "spotlight_local_search",
    "headers": {
      "Accept-Charset": "utf-8",
      "X-Apple-UI-Scale": "1.000000",
      "X-Apple-CachedResults": "[]",
      "X-Apple-UserGuid": "267af341-df6c-4eed-5e78-a2b8a49a1d1f",
      "User-Agent": "(OS X 14A389) Spotlight\/916",
      "Accept-Encoding": "gzip, deflate",
      "X-Apple-CachedQueries": "[]"
    },
    "type": "search_remote",
    "timestamp": 4188,
    "delta": 326,
    "url": "https:\/\/api.smoot.apple.com\/search?q=Spotlight+Inception&locale=en-US&time_zone=America\/Denver&calendar=gregorian&key=andromeda"
  },
  {
    "results": [
      "other"
    ],
    "timestamp": 5349,
    "input": "Spotlight Inception",
    "type": "render",
    "trigger_timestamp": 3814
  },
  {
    "delta": 0,
    "timestamp": 5553,
    "input": "Spotlight Inceptio",
    "type": "search_local",
    "ttfb": 0,
    "latency": 41
  },
  {
    "latency": 352,
    "input": "Spotlight Inceptio",
    "ttfb": 352,
    "delta_ref": "spotlight_local_search",
    "headers": {
      "Accept-Charset": "utf-8",
      "X-Apple-UI-Scale": "1.000000",
      "X-Apple-CachedResults": "[]",
      "X-Apple-UserGuid": "267af341-df6c-4eed-5e78-a2b8a49a1d1f",
      "User-Agent": "(OS X 14A389) Spotlight\/916",
      "Accept-Encoding": "gzip, deflate",
      "X-Apple-CachedQueries": "[]"
    },
    "type": "search_remote",
    "timestamp": 5868,
    "delta": 315,
    "url": "https:\/\/api.smoot.apple.com\/search?q=Spotlight+Inceptio&locale=en-US&time_zone=America\/Denver&calendar=gregorian&key=andromeda"
  },
  {
    "results": [
      "other"
    ],
    "timestamp": 5878,
    "input": "Spotlight Inceptio",
    "type": "render",
    "trigger_timestamp": 5510
  },
  {
    "latency": 406,
    "input": "Spotlight Inceptio",
    "ttfb": 406,
    "delta_ref": "spotlight_local_search",
    "headers": {
      "Accept-Charset": "utf-8",
      "X-Apple-UI-Scale": "1.000000",
      "X-Apple-CachedResults": "[]",
      "X-Apple-UserGuid": "267af341-df6c-4eed-5e78-a2b8a49a1d1f",
      "User-Agent": "(OS X 14A389) Spotlight\/916",
      "Accept-Encoding": "gzip, deflate",
      "X-Apple-CachedQueries": "[]"
    },
    "type": "search_web",
    "timestamp": 5922,
    "delta": 369,
    "url": "https:\/\/api.smoot.apple.com\/websearch?q=Spotlight+Inceptio&locale=en-US&time_zone=America\/Denver&calendar=gregorian&key=andromeda"
  },
  {
    "results": [
      "other"
    ],
    "timestamp": 5922,
    "input": "Spotlight Inceptio",
    "type": "render",
    "trigger_timestamp": 5510
  },
  {
    "results": [
      "other"
    ],
    "timestamp": 7131,
    "input": "Spotlight Inceptio",
    "type": "render",
    "trigger_timestamp": 5510
  },
  {
    "results": [
      "other"
    ],
    "timestamp": 8144,
    "input": "Spotlight Inceptio",
    "type": "render",
    "trigger_timestamp": 5510
  },
  {
    "timestamp": 23933,
    "input": "Spotlight Inceptio",
    "type": "abandonment",
    "method": "lost_focus"
  },
  {
    "timestamp": 23933,
    "input": "Spotlight Inceptio",
    "type": "abandonment",
    "method": "close"
  }
]HTTP/1.1 200 OK
Access-Control-Allow-Origin: *
Content-Type: application/json; charset=utf-8
Date: Sat, 18 Oct 2014 04:58:51 GMT
Content-Length: 3
Connection: close

""

1_keystroke_post.txt

GET /search?q=Spotlight+Inceptio&locale=en-US&time_zone=America/Denver&calendar=gregorian&key=andromeda HTTP/1.1
Host: api.smoot.apple.com
Connection: keep-alive
Accept-Charset: utf-8
Accept: */*
X-Apple-UI-Scale: 1.000000
X-Apple-CachedResults: []
X-Apple-UserGuid: 267af341-df6c-4eed-5e78-a2b8a49a1d1f
User-Agent: (OS X 14A389) Spotlight/916
Accept-Language: en-us
Accept-Encoding: gzip, deflate
X-Apple-CachedQueries: []

HTTP/1.1 200 OK
Access-Control-Allow-Origin: *
Cache-Control: private, max-age=300
Content-Type: application/json; charset=utf-8
Date: Sat, 18 Oct 2014 04:58:33 GMT
Content-Length: 436
Connection: close

[{"status":"NO_RESULTS","query":"Spotlight Inceptio","prefix":"Spotlight Inceptio","completion_score":189999,"fbq":"eyJ1IjoiMjY3YWYzNDEtZGY2Yy00ZWVkLTVlNzgtYTJiOGE0OWExZDFmIiwicCI6IlNwb3RsaWdodCBJbmNlcHRpbyIsInEiOiJTcG90bGlnaHQgSW5jZXB0aW8iLCJ0cyI6MTQxMzYwODMxMywiZyI6InVzL2lsbGlub2lzL2Nvb2svY2hpY2FnbyIsImEiOiJzcG90bGlnaHQiLCJkIjoibWFjIiwibCI6ImVuX1VTIiwiaSI6IjE0MzQ0MS0xLDIxIiwiYyI6IjQxLjg2NDMsLTg3LjY0NSIsImVjdiI6MCwiY2N2IjowfQ=="}]

The big strings seem to be base64 encoded JSON.

e.g. if we take fbq from that last line:

require 'json'
JSON.parse \
  "eyJ1IjoiMjY3YWYzNDEtZGY2Yy00ZWVkLTVlNzgtYTJiOGE0OWExZDFmIiwicCI6IlNwb3RsaWdodCBJbmNlcHRpbyIsInEiOiJTcG90bGlnaHQgSW5jZXB0aW8iLCJ0cyI6MTQxMzYwODMxMywiZyI6InVzL2lsbGlub2lzL2Nvb2svY2hpY2FnbyIsImEiOiJzcG90bGlnaHQiLCJkIjoibWFjIiwibCI6ImVuX1VTIiwiaSI6IjE0MzQ0MS0xLDIxIiwiYyI6IjQxLjg2NDMsLTg3LjY0NSIsImVjdiI6MCwiY2N2IjowfQ"
    .unpack('m')
    .first
# => {"u"=>"267af341-df6c-4eed-5e78-a2b8a49a1d1f",
#     "p"=>"Spotlight Inceptio",
#     "q"=>"Spotlight Inceptio",
#     "ts"=>1413608313,
#     "g"=>"us/illinois/cook/chicago",
#     "a"=>"spotlight",
#     "d"=>"mac",
#     "l"=>"en_US",
#     "i"=>"143441-1,21",
#     "c"=>"41.8643,-87.645",
#     "ecv"=>0,
#     "ccv"=>0}

Damn, that "c" parameter in @JoshCheek 's parsed result is the location of the user. Meaning Apple also tracks your location every time you type into Spotlight. O_o

As I recall, I was asked if I wanted to allow Spotlight to access my location. You can disable it in System Preferences: Security & Privacy : Privacy : Location Services : System Services