Water inlet

Hai Alfagemeinde, maybe one of you can help me. I drive an Alfa 156 1.8 TS, Built 98. He has now about 104Tkm down and ran until now without major problems. Last August the flex pipe was changed on the exhaust and the workshop of my trust still said how good he is in shot and has no rusting approaches. By chance my wife, who was with him outside, discovered that everything was wet in the footroom back. The water was about 2cm high (only behind the ride). She had to cut the whole cover and felt, which was already completely porous, to drain the water off. I had it now on stage. There is absolutely nothing to see where the water could enter. No rust. Except a few sweaty water stains, from where the water is dripping down now, nothing is to be seen. Advice boxes etc. everything is fine. At the moment a mystery from where the water could come. Someone has s chon had such a similar problem and if so, what can you do about it. Thank you. Greetings from Slazenger156