A few years ago I was at Manchester Airport waiting for a flight, browsing in the WH Smith there I heard a family behind me, the gist being that the little lad (about 6 or 7 years old) had lost his spending money for the airport, mother saying he should have taken more care and couldn't have any more, kid crying. I did look round and see this family.
They went and I continued browsing the shelves and looked down and saw a folded up £10 note. I immediately thought of this kid, picked up the money and went to see if I could find them. I saw the father and said "Excuse me, I overheard that your child had lost some money and then I found some in Smith's", he called his son over and I handed it back to him, he was so happy.
I don't think I could have kept that money had I not found them, I'd have most likely put it in a charity box - it felt so much better handing it to the lad than the feeling of finding £10.
Same trip, in Madrid, my GF found a €50 note and felt bad keeping it as someone had lost it, so we gave it to a homeless person.
However, I also once found £5 in a supermarket trolley, I kept that.