Dear forum members, I have ran into similar problem like described here. Car is Audi A4 B8.5 2014 2.0 TDI (130 kW), no sun roof, not involved in accident.
Problem discovery:
Couple of weeks ago I left my car at Audi dealer for battery replacement. Day after I took over the car I noticed that boot lining on the boot floor is wet. Since the wet spot was close to the left tail light I have opened the boot compartment where I have my AMP and the sponge was also wet. My initial thoughts were that water was spilled in the boot. I have also checked the spare tire compartment and there was no moisture. So I dry this sponge and AMP compartment and while drying I also wiped the wax that Audi puts inside of these compartments because it had some dirt in it (later I read that it is not uncommon to have dirt there due to the cabin pressure vents). Beside the dirt, compartment looks clean, no rust or anything.
Troubleshooting:
Part 1
Although I suspected that water was spilled in the boot, I decided to overflow the rear of the car with the water hose and check for any leaks. After 20 minutes of soaking there was no visible water ingress into the boot or the AMP compartment. After hose test I power washed the car and it was again dry. This led me to conclusion that assumption of spilled water was correct.
Part 2
So I concluded all is OK and I drove the car. Here and there I checked the AMP compartment for moisture. There was no rain for a week. After the first rain from the initial incident I went to check the AMP compartment and again the sponge was wet. Then I did hose test again. Father was soaking the car and I was in the boot for 20 minutes and I could not locate any leaks - nothing! Then I asked father to spray the water under pressure directly to the boot lid side drain (not something you would encounter during driving) and I saw a couple of water drops coming through the cabin pressure vent top side. Conclusion was that the seals on the vent went bad. I have also checked the stoper rubbers and they are dry. Checked the main boot compartment seal and it seems OK but on the left side it is pretty loose (it comes of the lip very easily).
Part 3
Present time, car was on the rain over night and I check the AMP compartment before work, all dry. No visible ingress of water trough the cabin pressure vent, nothing.
It was raining the whole day I was at work so after the work I again check the AMP compartment and seems dry. While I'm looking into the compartment I see that water drops appear after 20-30 seconds after the boot lid was open through the small gap between two panels (marked on attached picture). Could it be that the main seal is loose and some water gets accumulated beneath the seal. While the seal is compressed by the lid its holding the water and when I open the lid, seal expands and accumulated water drains to the side panels and consequentially to the AMP compartment? That would explain the delay of 20 to 30 seconds.
I have never really looked into this AMP compartment until I noticed that boot floor lining was wet so I have no idea how long this is occurring. I also must say that apart from when i picked up car from the service, boot floor lining was never wet again. In parallel I was also checking the passenger side compartment and that one is always dry.
Car was never involved into an accident. The issue is driving me nuts!
Any ideas or comments on what I am facing here?