I'm thinking he means "hidden" as in the cars/cameras parked on the side of the road with a sign warning you AFTERWARDS that you've just passed through it.
In NSW they have plenty of signs BEFORE all of their fixed speed cameras to warn you... "SPEED CAMERAS OPERATING THROUGH NSW", then "SPEED CAMERA OPERATING 24 HOURS", then "SPEED CAMERA AHEAD" and then the actual camera. Gives people plenty of a chance to slow down