Wet carpet behind driver's seat

I drove through a storm today and, when I got to my destination, noticed my floorboard behind the driver's seat was rather wet. I'm not sure it is due to a bad window seal as the water is rather concentrated in one spot. Could it be anything else, such as water coming up from the road? Additionally, this happened to be the first time I ever turned on the heater, so I'm not sure if that could have any impact on drains, etc. I can test the window seals w a hose/smoke test them, anything else before I call my mechanic? The car is LHD.