I am guessing probable cause. They got a report of lewd behavior. She and her boyfriend were having sex IN the passenger seat WITH door open and cleaned up afterwards with tissue according to witnesses. No decent woman does that in broad daylight.
What people allege they saw and what the police find upon looking into a complaint are two different things.
The article in the Daily Mail states it was people from a nearby building that complained. How far away were they? Were there any complaints from anyone who was nearby with a clear line of sight?
Notice no mention is made of the police questioning people in the area to see if the couple had been having sex in public.
Notice too that the police themselves state that no crime had been committed. Which means they had every reason to believe the couple were not having sex.
Working with your suspicions (guess), there are a few variables.
First if public sex and/or indecent exposure are criminal acts under the local code, then a citizen's report of those activities constitutes Reasonable Cause for investigation including identification of persons fitting the physical description in the citizen's report.
If the citizen's report is overly vague, lacks a description, or is anonymous, then much more care must be taken and much less Reasonable Cause exists, and there may not be enough Reasonable Cause to justify anything more than observation.
If there is no statue adressing the Citizen's complaint (no law on the books), then there is no Probable Cause whatsoever, but police are free to respond and observe the area only on a general patrol basis alone (walking their beat, so to speak).