I'm not sure, but next time you're nosing around up under there, be on the lookout for something possibly like this:
On several occasions, and in at least two other Honda products, I've experienced major passenger footwell moisture as described. In all those cases, there was a condensation drainage hose which had become disconnected from its source by an errant passenger foot, etc. The hose normally connected to part of the weather machine, I suppose the heater-evaporator assy. or some such thing, and was attached to an outlet port in said housing with a slip-on fitment. The hose then routed to outside the cabin for drainage of the condensation. It was rather easily knocked off, after which point operation of the AC system steadily drained water into the passenger footwell. I can't recall noticing the last time I contorted myself into that footwell whether the NSX incorporates such a hose or not. I can only remember that it was a painful experience and I'm not up to it tonight. Anyway, if you'd been operating the climate control between the time it was dry and the time it was wet, I'd take a flashlight and scan up under there for a possible open drain port / disconnected hose. In my cases the hose was black 'rubber', maybe a half inch OD. Honda tends to do things very consistently (and usually very well), and I wouldn't be surprised to find a similar hose up under the NSX dash. If it's not an exterior leak from the washing, then this is a good guess.
- Richard
'93 Blk/Blk