If the previous owner had cats, in the same spot, where urine could get on a wall, it could be the reason.

They say when a cat pees on a vertical surface, that they are marking their territory (I could be wrong, but this is what the vet told me).

Maybe repainting that area after a very good cleaning. If that don't work, you may need to replace the drywall in that area. I don't mean the whole room, just the affected area.