Going back to the original post before all this testerone starting flying around, could they have been from the DVLA and checking number plates to check vehicle tax?
I was having a snooze in a layby when a face appeared at the window. The guy asked about my fuel and said he would like to dip the tank. (I was driving a Peugeot 504 at the time -- not worth stealing). "Costumes & Exercises", thought I and said to go right ahead. They did their stuff and gave me no cause for suspicion. They were in civies and driving (as I recall) an oldish red Ford van.
I now wonder if they were a couple of chancers checking the vehicle to see if it had been abandoned! Must phone HMC&M to see if they do this sort of thing in unmarked cars. Just a thought.
Derry