A British tourist riding on the worst metro line left her (expensive) phone on the seat and went behind the seat to look at the metro map. Even stayed there while the doors were open and at least me and three more people could have easily snatched it and left. I don't think the subway in the UK is much better so not sure what she was thinking.
I was on a train in sweden, and at a stop this guy just went and forgot his phone (no backpack or any kind of things left, just his phone), I tried to hail him but that didn't work, so I took the phone and raced after him, giving it to him outside the train.
He was talking to someone and was clearly not happy about it all.
Well well, I boarded the train and thought I had at least done a good deed.
Guy comes aboard and rides the train to the next station...