I know a famila which doesn't use deposit carts, and they happen to share a parking lot and cart pool with an Aldi which also don't use deposit, a famila employee does the corralling -- mostly re-distributing carts between isles as people do, in fact, return carts just unevenly so.

I don't really think it's about the deposit, culture-wise, Germans are as likely to understand a deposit as "that's mine now", see Christmas market mugs. It's signalling "please really do return carts it's important we don't want to hire someone to do it and bill you for it that would make our milk 1ct more expensive than the neighbouring store".