Passenger floor is wet

2002 Voyager with front passenger carpet wet with white gunk between the
seat and the door. My mother-in-law asked me and I have no clue. Only two
things I can think of: leaking when raining or heater coil. I sprayed a
hose on passenger side door and windshield and observed no leakage. I
checked her drain holes and they were not clogged. Any educated guesses or
answers.

Thanks