This is a picture I took while visiting Copenhagen in 2023. Unlike most of my posts, this is not as directly about a (literal) sign, but I feel that the camera here is a strong sign in the more general semiotic sense.
Indeed, the camera has multitude of purposes. It is not only surveilling the door, but by not being a hidden camera, it also very clearly signals by its positioning to that this door is being monitored. Note also that the camera does not seem to have a very wide field of view: the camera sees nothing but this door.
"I don't care what you do to the wall, but leave this door alone!"