Agreed about having to field red herrings - but I can't reveal the exact challenge - as it is not illegal, but would be misunderstood by the parties being surveilled...
Inductive sensing is out - as already mentioned, because the sensors can't consistently be in a position near enough to the target object to be sure of a field.
Some may be viable, but >90% no chance, plus civil works to dig-in a loop would not be an option.
Visual-only is out - because some detection may take place in near blackout conditions... vehicle arrives, parks, engine off, but still in position to be detected as a warm mass.
PIR can assist with this. Perhaps not full field PIR, but a defined matrix of points with credibility score to return a pass/fail.
The vehicles - for clarification are typically regular passenger vehicles or small commercial vans.
Parked in reserved spaces, at the side of the road, or in other outdoor places.
Sensor assemblies must be as small as possible for covert placement on *any* axis wrt the sensor unit between 2-10m line-of-sight from the actual target vehicle.
Goal is autonomous operation on solar charged cell pack, with 'simple' low-power mesh comms of ID & status/score data as needed.
Currently this is a concept paper being developed, to be followed by some R&D and a POC if it seems viable at the target price point. All your ideas are appreciated - so please don't think it's a waste of effort. To me it's a really interesting challenge to solve a real-world problem that is not in a perfect / controlled environment. In fact if it looks do-able, it could easily become a crowd-funded project.