If you found a $100 bill on the ground and kept it, is it considered stealing?

No, if it is just lying there on the ground, it is unclaimed and therefore you wouldn't be stealing it from anybody. If it was a wallet with $100 in it, it would probably have identification in it as well, and so the standard of care would mandate that you contact the person whose identification it was. A lone $100 bill, however, comes under FK(Finders, Keepers).If the ATM mistakenly gave you an extra $100, there would be bank records and your account would be debited that amount extra.