I found a 20 at the bank I took it?

Donate it to charity... Karma

Cash is unlike a card because there is no means of identification on it. Sure, cameras saw you, but how would they be able to enforce various ethical property values on this item?

I wouldn't be scared . if someone really wanted that 20 back or was paying attention they would have came back for it

Finder's keepers.

buy yourself a happy meal and be happy! :D

if it was on the ground and no one else was around, it's yours

Was it the banks ATM or an off site ATM? Off site you may as well keep it as you don't know what bank was accessed. If it was a bank ATM, (yours?) take it to the manager, you will feel better.

If it is outside the bank, it is yours. Someone probably dropped it at the ATM and didn't notice.

Hmmm...I would say, march to the bank immediately and take the bill back. Explain to the tellers exactly what happened:"I came to the bank the other day to access the ATM and found this bill. The bank was closed so I took it home, but came back as soon as I could." Otherwise it would look like theft and I am sure you meant best.
Hopefully they understand. Best of luck in this difficult situation!

They will find you