Now I can confirm: It was the Greg sign. I don't know whether this is a bug or a feature though.
I built my own house on the empty lot. I was just wondering whether the „Beware, Greg!“ signs (I don't know the actual name of the object), which you can add as decoration and I recently added in the garden, since a werewolf family lives in the house and I like the looks of it, might cause the bug. Maybe the game does not realize that placing the signs somewhere does not mean Greg is nearby and, thus, triggers moodlets you'd usually get around him. I'm gonna playtest it later today. But either way that would still be a bug, right?
I'm gonna test that when I'm back home. However, the signs are no debug object, right? So it would be a bit weird if you cannot place a standard deco item (it is literally found in the decoration section if you filter it by pack) on your lot without that having negative consequences on your sims, especially with no information on the object description.
