Similarly, when travelling by ferry or train from UK to France, it was a common occurrence to be pulled over by a police officer (on the UK side). The usual routine was for the officer to put on a pair of nice white cotton gloves and run his hand all over various surfaces of the car, inside and out. The gloves would then go into the office and into a machine for analysis. One doesn't see this sort of thing any more.
Carried out by civvies rather than plod but it's been routine last few times I've used Chunnel or ferry.