You're still ignoring the fact that we really do not know why the police were concerned.

You don't know what kind of activities were going on that night that caused them to be concerned, and it's not necessarily their place to explain it to you.

You don't know what might have happened in that parking lot in the past that would cause them concern. For all you know, someone got hurt their as a result of gang activity or street racing/show off activities, or the store might've been robbed recently.

The common law concept of "malingering" or loitering goes back a very long way, and is related to the ancient practice of highwaymen waiting in ambuscade.

In consequence of this ancient concept, localities are allowed to regulate malingering and loitering. If you are perceived to be just standing around waiting for something to happen, for someone to pass by, then a locality has the ability and right as a corporate body of subjects to regulate that lack of activity.

If you're there for a specific purpose, which is to say you're waiting for so-and-so who's supposed to meet you there at such-and-such a time, then that's a different story.

I honestly believe that they were being careful because of activities of which you're just entirely unaware.