It Has To Be A Rat
The simplest explanation is always the most accurate one and the one that solves the problem. In this case, it was the rat. Rats can get stuck inside the walls and move around just a bit. Others can move or even run inside the walls which would cause noise.
This was the only explanation at this point and seemed like a legit one. The owners have decided not to wait one day longer and that they will need to solve this today. It is time to act and solve the problem.