[A4] Water leak in an A4

[Sean Paul]

My rear passenger floor had an inch of water in it recently after a rain.  No leaks detected through the window or door jambs, and only the floor was wet.  It's also very hard to tell if it's still getting water - it's just hard to dry thoroughly with no garage.  Ideas?

Also of note the front compartment between the winshield and where the engine is sealed off had standing water.  I did unclog the holes there (one under the battery and one on the driver side) and that water drained. I thought that might be related but the mechanic says no, and it likely isn't related to the heater core.