Anyone got any idea on what tolerance these vans operate at?

I know the ones in Dyfed Powys are set at 40 in a 30mph area.

Reason I ask is that I was sitting at just below 80 on the M4 near Bridgend on Saturday morning (definately around 78 and less than 80), came over a brow and there it was.....sitting on a bridge.

My Bel Euro 550 didn't even flinch...no beeps, nothing (I know that detectors give little prior warning for laser but surely if I was lasered directly it would have beeped???)

What do ya reckon guys?

Dave