Strange RF detector signal

[dotnw@xxxxxxxxxxx (Dot net work)]

Hi. I just rented out a new place to live, and used an RF detector to
scan the whole place. No signals were detected except one very
specific area in the utility room.

Strange thing is, the detector lights up in one small area, about 2
feet away from a wall. When I move the detector closer to the wall,
the signal weakens, then disappears.

In other words, the signal that is being picked up seems to be in
"thin air" and not from any physical object.

Any ideas very welcome.

Thanks a lot, regards from dnw.