Water in passenger footwell
If you've noticed that the passenger side footwell is wet or damp you might have a problem with a blocked drain hole. This is a common issue where the drain hole just infront of the pollen filter gets blocked with mud and debris and water builds up, finding it's way out through the heater system etc into the passenger footwell.
What to do:
You can find the drain hole under the bonnet beneath the windscreen area, if you can see it's blocked you can just poke a screwdriver in to clear out the debris. You'll probably also need to replace the pollen filter which would have been ruined by the water, these are pretty easy to replace.