From what I remember, they give off no signal until trigger is pulled - which they do when pointing at subject car.
It's a straight beam, and if not pointing at you, have to be lucky for it to bounce at your car/detector.
Something like that.

Driving a hgv is worrying, some read them as speeding when they're not moving

I used to have a snooper, but it went off all the time at nothing.
You get to recognise prime places for a van/trap though.
I got caught on a 30 road that few do under 40 on, so obviously an innapropriate limit, and good pickings for a trap.

Had a close one recently 200 yards from a gatso camera, was a van camera set up.
Bit devious that