There is a tray around the under side of the sunroof designed to catch any water getting past the seal. You can see the tray with the sunroof open. There are 4 drains connected to this tray 2 drain forward and 2 out he back(Pano roof may have 6 not sure on the pano). That said these drains can only take so much water and as we know water migrates to the lowest point. If there is too much water in the tray and it goes to a plugged drain it will spill over into the vehicle. Whats been suggested above is you get into a dry area open sunroof take a bottle of water slowly pour the water into say the right front corner, you want to see the water pouring out the right front drain. if its not running out of there at a decent rate or not at all good chance that drain is plugged. If you look up checking/clearing sunroof drains I'm sure you will find a good DIY. In my case the sunroof seal was simply not sealing any more and allowing too much water into the tray and my drains couldn't handle the volume. In my X5 I can hear a lot of wind noise coming from the sunroof area so I'm confident its time for a seal. After doing the seal in the 7 the noise in the cabin was greatly reduced and more importantly no leaks.