Putting the cart back when nobody would ever know is a tiny choice with a big answer in it.

Small actions with no witness are the only honest test available. There is no credit, no consequence and nobody to perform for, so whatever you do is just what you are like. It does not make you a good or bad person on its own. It tells you which direction your default is currently pointing.

Watch one of these today without steering yourself. The cart, the dropped wrapper, the extra change you were handed. Then, if you want, do the other thing once on purpose and see how easy it turns out to be.