Even if drains work fine, you may still have leak through seals or also behind the seals. Seals shrink and I guess the butyl strips that hold them to the car are letting go. I would check two areas: seals above rear window on top in the corners, where they are curving/bending. If that's leaking, you might not even see it because it will go over the plastic panel right behind the cloth thing near rear headrests and you will find water somewhere in the back of the car and also under the rear seats (just pull them up and check, they are easy to remove).
Another area to check are those drains shown in this thread in the last post:
https://www.e90post.com/forums/showt...3#post25393023 there is visible drain hole there with an outer ring and there is a drain coming from the roof with a plug with needle-like metal spring loaded valve that goes into it, when its all closed, the needle is pushed in and that is supposed to open the valve.