Well, it was common when I was a housekeeper back in the mid-late '80s. The customary tip was $1 per day the room got cleaned. About 3/4 of guests would tip that. Ironically, the ones who didn't tip were almost always the ones who left the room trashed, which was completely backward. Leaving a room in good shape was considered as good as a tip by my fellow housekeepers.
I didn't bother to work out what the German meant, but the "eco-friendly" thing where you don't want them to change the towels every day took the form of a small, woven bag. I hung it on the door, and was pleased that housekeeping didn't come into the room at all -- I don't like my stuff being moved around.
When I left I noticed there were chocolate bars, fruit etc in the bag, one item for each day I'd stayed.
But (at least then -- I've noticed that things are a bit different now) there is usually a little card telling you the name of your housekeeper. That's where you're supposed to leave the tip (if you're leaving one). Money on the card was deemed a tip, money left anywhere else was deemed forgotten property and turned in (if the housekeeper was being honest).
Where I worked (at a Hilton), we had to clean 17 rooms per day. If every room left a $1 tip -- which never actually happened -- that would come close to doubling my wages for the day. It made a significant difference.
By signs I mean things like you mentioned nudging one to tip. It’s not an overt sign stating it is expected but it is a sign letting you know that it is expected.