Turning it into the transit authority is probably to most expedient way for that person to find it, in theory anyways. Maybe put out a thread on Reddit detailing where you've left it and maybe one of the screen names on the system or something, post to Facebook and any other social platforms you can. But only if you're that invested in making sure the person finds it I guess. I wouldn't keep it, that's bad karma and it'll come back to you.