Worst thing you can do is leave, move or otherwise cause them to notice you..
I have always found, that when seeing a poilce car, flick the radio to Radio4, slip down in the seat and pretend to be doing 'dosing' or clearing texts off the phone, or even close one eye on the side they are approching from and stay still changing eyes as they pass.
Usually 1 of 3 things happen..
They see you sleeping as they drive by in thier headlights and leave you alone..
They will get out and approach your car, close both eyes and wait to see if they knock and if they do, make out that they startled you and rub your eyes. If you make out they really made you cr*p your pants, they normally apologise and walk off.
If they come over and ask what you are doing, simply say chilling listening to the radio. The reality is, unless they actually catch you with your pants down or staring into a parked car, they can do absolutly NOTHING other ask you to leave or move on and at the point, just say ok and leave calmly..
Many times, I see cars racing for the exits and either be stopped by a second waiting car to ambush them as they are flushed out, or seen the only car that leaves being chased.
Dont bring on attention, sit tight and if asked what you are doing, dont forget you are doing nothing wrong.
Dont be cocky, but if they start asking silly questions or making accusations, stand your ground and let them know that sat in a car in the middle of the night is not breaking any laws.
The number of times I have been told 'its a bit late at night to be going to shop at ASDA' to which I usually reply, 'strange why ASDA is open all night then?'
The other one is to be told that 'why are you at a dogging site' to which I normally reply, 'dogging, wow, I had no idea' or 'a dogging site, you sure as I aint never seen anything happen here, have you got the right place?' and my favourite 'dogging, is that why people keep walking over to cars that pull in and put the lights on? at least I know what they are up to now, thanks for letting me know'
'Getting some fresh air' is always a good answer to 'what are you doing here'..
Got caught with pants down last week and after some banter with the copper, just told us all to go home.. Suppose it depends who you get..