A marker means your van is put onto the police database and flagged up every time your van is pinged by a police car.
Trust me you don't want them to do that as you will be for ever stopped by the fuzz and given a hard time.
Yes, which is harassment.
A mate of mine bought a 2nd hand car from a local dealer and within a couple of weeks he'd been stopped by the cops 3 times. The third time the copper let it slip that there was a marker on the vehicle (apparently previously owned by a druggie) and when he asked how it could get taken off the cop laughed and said "no chance mate!"
After a long and arduous battle with the police, he eventually got a solicitor involved who advised he sue the police for harassment, which he started to do.
Immediately the cops offered to settle out-of-court for £5,000 and to remove the marker immediately, Which he accepted.
They cannot use arbitrarily putting a 'marker' on someone's vehicle as method of punishment, and they know that any court would come down hard on them, as it is clearly harassment, and to threaten it is against professional standards.
This 'marker' is intended as an investigatory tool, not a punishment, and the police cannot use it as a means to harass people.
I would report that copper to the force's professional standards department for using the threat of a marker as punishment, as it's pretty outrageous and they are not supposed to do it.