I found a $20 on the ground at Starbucks when I was working there, and I kept it. Obviously, if any customer were to ask about it or come in looking, I would happily return it to its rightful owner. I don't see why in cases like this it's suggested that doing so would be dishonest. I would take a quick scan around the area, and see if it appears that someone may be missing their money. Maybe I observed that someone was just recently standing in that area where the bill was found. I would not feel the need to turn it into the business, because first of all, it's not their money. Secondly, it's not likely that someone would come back looking or even notice that it was missing or where it went missing, so the business would end up pocketing it (in which case, why should they get it and not me?)
On the contrary, one time I was walking behind a lady that had a $5 bill drop from her opened purse. She didn't notice. I picked it up, and gave it back to her.