Should have bought a GPS based "Accident Blackspot" detector. The ones that recieve Radar go off every 5 seconds. Anything can trigger them from car key fob unlocker things, house alarms, car alarms, traffic lights, radio masts oh and speed camera's. Thats why I slung my old snooper. Even in "city mode" it went off all the bloody time. So I bought a GPS Road Angel unit. Update it via the web every morning and it will only make a sound near a camera site (even mobile) and it detects laser.
I think you should have looked here before posting
http://www.originb2.com/product/features.cfm
The reason people buy units with extra radar and laser detectors is because the GPS ones rely on somebody updating them. So they are great for fixed posts. If, however, plod are in their vans doing random areas (as they do round here), or are following you in an unmarked car, a unit which is solely GPS based is as much use as a chocolate teapot. As is any laser detector because unless you are really lucky and pick up a reflected or refracted beam you are too late to do anything about it. Once the laser has pinged it's game over, you can slow down but it won't help! Because the laser is focussed, you won't get any warning (with the above exceptions) until they target you. The only solution is a laser diffuser (commonly called a jammer) which opens you up to a gazillion different charges, some of them nasty. I don't use any of them, if I am caught speeding I deserve to be fined. My opinion will probably change if and when I am "done" for the first time.
To get back on topic, just a guess but it's possible that there are traffic flow sensors in the area so that those in the know have an idea of the flow rates electronically (rather than the little blue cameras you sometimes see, usually high up or on bridges. They usually use induced loops unde the road for that sort of thing though.
I suspect that it is, like others have suggested, some byproduct of all the gizmos at LHR. The locations are probably arbitraty. The radar could be bouncing off a plane, off somebody's car roof, onto a road sign and then on to you (bit exaggerated but you see what I mean)