O.k. so then back to the marten problem:
We live at the edge of a forest and have done so for more than 20 years. Martens are not only protected but also hard to catch.
If you release them, they often come back.
Apart from that, it could also have been a hazel dormouse or a garden dormouse.
Since the type of animal was not clearly identified, it is difficult to suggest a course of action.
Basically, you should first talk to the forester.
Before that, however, you should already check where the new resident has entered.
The insulation used nowadays can easily be gnawed through by such an animal. Besides, these animals can scurry up house walls. Then you have to be consistent and install a cladding. Still, entry is not ruled out. The animals can literally thin themselves out.
Therefore, you have to spend money.
Accusing a construction defect right away is the wrong approach!
It is like with a woodpecker in the house wall: annoying and frustrating but it is nature and not a construction defect!