It boils down to force Policy, that's all. Some cameras are painted yellow to be seen, and they can recover a certain percentage of revenue from fines. Same for camera vans. Some will put signage out, some won't. Some will actively declare where they will be on their website beforehand. There's no hard and fast rule about being sneaky. On a side note, a few weeks back I was shooting down the A14, towards Cambridge from BSE and couldn't quite believe what I saw on the opposite side slip road. A marked police vehicle almost buried totally in a hedge with a copper trying to do a sniper spot with his speed gun. Even as a copper myself I thought it was bad form and was appalled. If your gonna do it as a marked unit, then do it and be visible. Leave the sneaky stuff to the unmarked patrols.