Condensation

Water vapor from the air won’t blow under the pad.
Quite right, definitely.
But it is not vapour blowing under the pad, it is water vapour condensing on the cold floor next to the pad, and that liquid water then flowing under the pad by gravity.

Matt’s experiment with ice in a ziploc bag will show that every time.
Um – unless the ambient air is even colder than the ice!

Cheers

hmmm….  maybe…

Jerry is correct, condensation under a pad come from the ground.

In all my decades of backpacking I’ve had this problem only under my tent floor, not under my mattress.