Nigelious

I drive S3 2001.i've never had any problems until Sunday. I drove through a quite deep puddle accross the road (slowly, carried on for another 10 mins, parked up, returned to the car about 3 hours later and couldn't engage any gear when the engine was on. Turned engine off and could engage all the gears. The clutch pedal didn't feel any different at any point. RAC turned up, checked the hydraulics and said there seems to be plenty of pressure in there, so it's probably the clutch. Dropped it at a garage for it to be looked at and when they started it in the morning, it drove perfectly and it has since.
Has anyone ever heard of this before? I'm driving it around still as i don't want the expense if i don't have to pay out.
So if anyone has any ideas that would be great, thanks