I wouldn't worry, I had an AAs410k on a bipod on the roof of my car while I zipped my jacket up at the exact moment the police went around the roundabout on the entrance to the grounds I was shooting on.
They came back around the roundabout, stopped and asked what I was doing. I showed them my permission slip and they didn't even get out of the car. Wished me a nice evening and said they would let the station know my cars location in case of any calls. (I actually call in now if I go to the same spot just to save them some hassle)

Regards
Rhys