Could be just about anywhere....water in the vehicle, a tarp over it, moisture would be everywhere......the floor was not wet when parked? Did the tarp have a hole in it?
If the carpet is soaked, it should be completely removed from the car, and air dried in the sun or you will develop a mold smell.....
With the carpet out, you can then have someone wet down the vehicle with a hose, and then pinpoint from the inside where the leak is coming from....