Wet floorboard

Many people have, and have traced this back to a leak at the third brake light. Water enters there, runs down the back of the cab, over the floorboard, and pools in the low spot, which just happens to be the passenger floor area.

If you notice it wet in the morning, after some rain, that's probably it. If it's getting wet without any rain, but you're running either the A/C or the defroster, then suspect a plugged A/C evaporator drain.