Thanks @TerryPG!
The purpose, to avoid graffiti, did not always work 
I saw also some of them destroyed.
I noticed those paintings the first time, when passing a hairdresser on Mondays, when they are closed:
I saw later, it was not the only one, but the challenge was, to find others, because you don’t notice anything as long as the stores are open. After getting “infected” to make more photos, we went searching on Sundays when many stores are closed.
Now, I wondering if I should start a research on Maps to find out what happened to those places meanwhile 