Letter overlayed over canvases

Trying to set up an arcade with lots of posters, for some reason this specific spot always has an S overlayed over the canvases. I remember having a similar issue a few years ago but I haven’t played in a while so I can’t remember if I can do anything about it.

What’s on the opposite side of the wall?

Nothing, it’s just water outside one of the satellite condos in the underwater one.