A lot of the time with Mobile Radar Cameras they are very low powered unit's.
So by the time your Radar Detector has picked up the signal you are already in the Camera range, and by the time
you have reacted to the Detector the Camera already has your Happy snap.
The Radar Detector Detector works by "looking" for the specific oscillator frequency that the radar detector use
to operate, Radar detector manufacturers have put a lot of time and money into masking the signal or shutting down the
Radar Detector to eliminate the emissions from the Detectors oscillator.
To Quote from Wikipedia:
Radar detectors are built around a superheterodyne receiver, which has a local oscillator that radiates slightly.
It is therefore possible to build a radar-detector detector, which detects such emissions (usually the frequency of the
radar type being detected, plus about 10 MHz for the intermediate frequency).
Some police radar guns are equipped with such a device.
I remember the Cairns Traffic branch made a big song and dance when the got there Radar Detector Detector not long
after the Radar Detector laws came in, they don't use it any more (a work friend of my Wife's, her Husband did a short
stint with the Cairns traffic branch while he was a rookie) he confirmed they don't use it anymore and they use the Laser
speed detection guns, they look through the sights, push the button to activate the laser and if they see the front of the
car dip the a second or 2 after the laser activates or the signal gets suddenly blocked they assume that you have a detector
and he said that 99 times out of 100 they were right.