Alternative implementation for _find_caller_plugin_file() (2nd Attempt).

snippet-from-class-freemius.php

<?php

private function _find_caller_plugin_file() {
    $bt                      = debug_backtrace();
    $backtrace_entries_count = count( $bt );
    $active_plugins          = get_option( 'active_plugins' );
    $active_plugins_paths    = array();
    $plugin_file             = null;

    foreach ( $active_plugins as $i => $relative_path ) {
        $active_plugins_paths[ $i ] = realpath( WP_PLUGIN_DIR . '/' . $relative_path );
    }

    for ( $i = 1; $i < $backtrace_entries_count; $i++ ) {
        if ( ! in_array( $bt[ $i ]['file'], $active_plugins_paths ) ) {
            continue;
        }

        $plugin_file = $bt[ $i ]['file'];

        break;
    }

    return $plugin_file;
}