Does that also apply when the cashier gives you too much/not enough return cash?

A while ago my to-go order from McD somehow included an extra cheeseburger.

Was that maybe a "present" by the company to a loyal customer? Should I have gone back and paid for a cheeseburger I never ordered? But if I give it back to them, they would just throw it in the trash anyway because they can't sell it to another customer.

I did this with an entire rucksack full of groceries once when I was tired and used the self-checkout. When I came back the cashiers expressed surprise that I even did!