Under those circumstances I would be VERY suspicious of it being a cowl overflow down into the vent box.
Ever clean the drains out?
They are between the ends of the cowl and the fender, and if those get plugged a hard rain can overflow the inlet for the vent and end up on the passengers floor.
The drains are just a couple of openings where the two pieces of the cowl come together.
If you look close, does it look like the inside of the vent box has had water in it?
This is what one person did to get the collected junk out of each side, but the factory drains are where the arrow is pointing.
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