Water in my X5 - Help!

[moose500]

Living here in CA, we are finally getting some rain. However, I noticed a pool of water in the back of my 2003 X5 3.0. From the back of the car with the tailgate down, it was in the right-hand (passenger) side compartment in the "Trunk." When I pulled the panel off, there is really nothing behind there except wires but when I looked all the way down, there was a pool of water collecting at the bottom! I got it all dried off but have no idea how it got there and if it will come again. Does anyone have any experience with this? Please help...

Thanks,

Mike
SJ,CA