Of course you shouldn’t have. I wouldn’t judge you if he’d already walked out and then you picked it up and kept it though. I saw a $50 note drop to the ground in the mall the other day and ran up to the person I thought had dropped it. He said no no it isn’t mine it must be that lady’s, pointing to a woman walking in front. I asked her and she said yes thankyou so much I’m very lucky that’s very nice of you etc. Now, since I didn’t actually see her drop it, I really hope she was telling the truth, but at least the man was honest enough to admit that it wasn’t his. It made me smile :)