* Everyone was at the original bar at the same time - someone else had photos from the night that had her and other people in them, and those people (her friends) could be found online.
* The bar had some kind of "sign-up for x" list, or some kind of contest, or some kind of photos of the night, which were helpful.
* You searched twitter/facebook/whatever for a girl posting about a boy she just met at bar X, blah blah
* We know she was going to some kind of event - you guys figured out what that was, and found a guest list or something.
* You guys did a some wi-fi sniffing and read a few emails. He did say he emailed you the photo, so you know which account to listen for.
Are any of those close?