The point about David and Betty parking way out on LHR made by welsh chapie a good one and sparked a thought . How many drive-in theaters were there in the area in those days ?
if a cop would see a single man in a car parked by a couple "getting it on" they would likely shew off the couple and likely check out the man thouroghly.
but at a drive-in theater he would be ok. And if he stalked couples in part for arrousal the drive-in movie was sure to deliver.
I and several dates were observed by men at our drive-in back in the mid 70's ,most notably my nexdoor neighbor and the owner of the drive-in.
Many single men and married wanting to be alone ,prefered drive-in's as it was more relaxing than a reg theater and you brought a 6-pack of beer,snacks Z may have scoped out couples getting busy at a drive-in theater, then if he saw the car again parked at say BRS or else where he knew who he had . he may have been a regular at them . also a lot of the movies were very action packed and violent.

