You will never get a 100% perfect warning system as police and councils will also use these databases to find spots that are not listed.
This is especially true for mobile sites, quite often they got reported because of a blitz for a few days - but for the rest of the year the police does not check there anymore.
Another thing is the lack of knowledge from some users:
Speed-, Radar- and Traffic-cams often look very similar, resulting in false alarms genrated by people that did not know the difference between a traffic and a speed cam.