Long story short. This is what he found after two visits and removal of my shower base plus digging up parts of my bathroom floor and wall:
This is the punction of the warm water pipe as such:
* If the shitty bonehead who had this fucking idea wants to complain about me insulting him or her, he or she may feel free to drop me a mail at ad001 (at) uni (dash) rostock (dot) de. I will most happily forward it to the apartment owner who in return will gladly forward all bills resulting from this idiotic try to avoid spending 50 euros for a plumber to fix an accidential puncture.
Yes, the "repair" was hidden under some adhesive tape. Pieces of it can be seen on the first picture.