unidentified probable leak

We moved into a new house about two and a half months ago. When we moved in, there was a water spot on the ceiling of the dining room. I am almost positive that it was not there four weeks prior at our inspection. It is located under a bedroom floor, but is within a few feet of a bathroom. We have used the bathroom (sink, toilet, and shower) several times. The stain never feels wet or drips, the drywall feels normal (not soft), and there has been no change in the stain's color or size. My husband says to just paint over it since we have no evidence it is an active leak and if/when it comes back, we'll call a plumber. What do you think?