Mysterious Dampness in Carpet

I have a mysterious, recurring damp spot in one room. There's no plumbing in that wall, and the floor on other side of the wall is dry. The underfloor is dry when I go into the basement and check. The room was closed all night so I know at least this time it wasn't the cats, and probably none of the previous times, either. There's no strong cat urine smell. (I've had cats long enough that pretty much any part of the carpet will smell faintly of cat when damp.)

There was rain and wind last night, but it hasn't done this in several months, during which period we've had much harder rain and strong winds.

I suspect that this occurs during just the right combination of rain and wind direction. However, neither the wall nor the ceiling (both painted plaster) are ever damp; just the floor (carpet, actually). Could rain enter through the roof, drip onto a part of the ceiling, run down between the walls and out under the carpet without dampening anything else?

I plan to have the roof replaced during the next year or two. I'll see if that stops the mysterious damp spots.