I was working with my good friend Cédric Terrier on this one and he told me he had seen similar interferences coming from motion detection sensor. BINGO, after that it took us a few seconds to find them. Indeed, two of them were installed in two adjacent rooms.
Here is what they looked like:
The Technology behind these sensors
Here is their antenna pattern:
All the information related to this product can be found in the following specification sheet document: