Tonight, as I was leaving work, I noticed that a bike that's sometimes parked near mine, was leaning against the rail opposite mine. It had a U-lock around the seat tube, but wasn't actually locked to anything. I might have stuck around for a bit to see if the owner showed up, but I had an issue with my own bike today, and was depending on a coworker for a ride home. I also figured that putting a note on it would bring attention to it, to the wrong people...so I just left it as it was.

Unfortunately, I work right near Fenway Park in Boston, and there was a game finishing up as I was leaving, so the streets were starting to get crowded. Hoping it doesn't/didn't get stolen...looked like a pretty nice bike. Jamis or Torker, I can't remember which...one of the two, though.

maybe it was a bait bike

I don't touch other peoples stuff. You don't know why they are doing or did something...maybe it makes no sense to you but it does to them and that is what matters.
Imagine locking the bike only to find out the rider forgot his/her key and can't ride their bike...or worse lost the key but didn't bother removing the lock...no good deed goes unpunished...

I would shake my head and ride off. When I was twelve I had a bike stolen (my stingray) Dad was pissed. I was a careless kid. Sometimes we learn the hard way. Then again, it could have been a bait bike, like was said earlier.

It would be bad if you locked the u-lock... and the other guy left it unlocked because he forgot the key/combination...

The U-lock wasn't unlocked. It was locked around the seat tube, but that was the only thing it was locked around.

Sounds like the owner went inside for something quickly and just didn't lock up.

Depending on whether the cops handle stolen bikes, maybe serial # + location, in case that might help?

But then getting to the serial # would just make passer by curious and think you're a thief...

Unlikely...I was out there for at least 10 minutes, since I had to take the wheels off my bike (bolted on, with a coaster brake on the rear...not particularly time-consuming, but moreso than with QR skewers), and wait for my coworker who was giving me a ride in his car.

Ignored it FTW. None of my business.

The guy might not have had the key for his own lock and hoped that no potential thief would notice. This has happened to me. I do my best to make the bike look locked. I have even forgotten my lock and lashed my bike to a rack with bungie cords. For a while, I couldn't find my lock, so I used a broken lock. Anyone could have inspected it and figured it out, but I've been lucky with this lately.

What could you have done?

If it's a regular bike it may not have been a bait bike. It could have been a genuine mistake on the owner's part.

Maybe you could have locked his bike with your own lock and put a note on that with your number on it asking that person to call you in order to open it.

That's a bad idea, though well-intentioned. What happens when the note falls off and blows away? The owner gets back and sees some jerk locked his/her bike with no way to remove it. If I was the type to leave my bike parked unlocked and unattended for a quick trip inside somewhere, and came out to see someone else had locked it without my permission, note or no note, I'd be absolutely furious.

The owner may very well have had a good reason for leaving it unlocked. IMO best to just leave it alone. My rule of thumb is I don't do anything to anyone else's bike without their explicit permission. About the only exception is to move it to the side of the road if it's left in the street, which has happened once with some little kid's bike. Probably got too excited when his mom called him in for dinner.

Many options about what to do in that situation, but I applaud you for your conscientious thoughts about the other guy's bike. Hopefully he was making a quick stop, and came out right after you left.

Mom taught me not to talk to strangers.....

I'll admit I did something like that when I forgot the key to my lock. Difference is I ran in grabed a few things then ran out the store.

Any U-lock I have used, requires the key to both lock and unlock.