how a detector van works
My dad used to operate a detector van so let me share the science with you.
Your tv is a recievet and not a transmitter. In TVA (analogue ones atleast) the tuner resonates at a fairly specific frequency range in order to pick up the signal. This is what the drtectir is looking for. They also have hand held detectors which can point to the signal with an accurancy of under 10 feet.
So the db proves you don't have a license and the detector proves your watching tv. This is good enough to push for a fine and more than enough for a search warrent.