As I got off a train in Manhattan on Wednesday, I paid little attention to a flutter out of the corner of my eye on the subway. Then another passenger told me that I had dropped some money.

“That isn’t mine,” I told her as I glanced at what turned out to be $90 on the ground.

I realized the flutter had been the money falling out of the coat of a man standing near me who had just stepped off the train.

The doors were about to close, and no one was acting, so I grabbed the cash and left the train. But I was too late. The man had disappeared into the crowd. I waited a few minutes to see if he would return, but he was long gone. I tried to find a transit employee or police officer, but none were in sight.

I was running late, so I left. But now what? What are you supposed to do with money that isn’t yours?