you're misreading the law, it's not that objective. the law doesn't say that if merely one or even several of these conditions are met then it's a clear case of prowling or loitering. while you can't tell from fifty feet that he's eating in his car, that doesn't mean that once you get closer and realize he is that you can go with your justifications of your observations at fifty feet.

it absolutely is "usual" for people to use temporarily disused parking lots such as those for places of public accomodation to do such mundane non criminal activities such as eat, take a phone call, look at maps, rendezvous for giving people a ride, etc. if the business wanted people not to use their parking lot for such common activities during non business hours it's incumbent upon them to erect a gate or signage to that effect.