Yes, the police have no right to ask you to get out of your car and open your trunk, etc for inspection, unless they have probable cause. The existence, though, of probable cause is difficult to asses; some cases tell us it's more than a mere suspicion but others have been more linient towards the explanations given by police. In the case of those hanging around places where car-enthusiasts usually gather, the cops might claim they were suspicious of the cars there and decided to make sure all were legit.

However, note that probable cause is only applicable to government agents, i.e. if a security guard forces you to open your trunk while going to the mall parking and he finds some unlicensed firearm or drugs inside, for example, you cannot claim that the search was illegal since the one who found it was a private person. There's a case taught in law school involving courier service employees who found drugs in a box meant to be shipped and the court allowed the drugs to be presented into evidence even if the accused claimed it was inadmissible since there no probable cause for the search since the courier service employee was not limited by probable cause, that is reserved only for agents of the state.