Some folks I believe are under the impression that Trello and other apps like it are more private than they are. These are some Google queries as a proof of concept, that demonstrate the issue. Note: Need to look into the Trello security settings to see if there is a sufficient work-around.
inurl:https://trello.com AND intext:@gmail.com AND intext:password
inurl:https://trello.com AND intext:@outlook.com AND intext:password