Articuable facts and reasonable inferences which, based on the totality of the circumstances, would lead one to believe that a crime has been, is being, or is about to be committed.
No percentage likelihood has ever been assigned. However, the "preponderance of the evidence" is 50.1%. Probably cause
is lower than that, lower than 50-50 Reasonable suspicion is lower still.
In the OP: an area suffering a rash of car busts, commonly found at night. A non-resident in the area with items in his car such as are frequently stolen in car busts. A time of day where no business is open, the vast majority of people are asleep and he does not provide an explanation. He's armed. Yes, Virginia, there is RS.
Of course there may be innocent explanations, just as Mr. Terry could have ended up being an indecisive window-shopper. Doesn't matter.
And of course you can listen to any lawyer, who will tell you not to talk. And your night will get longer as we do the work ourselves, guided by our own suspicious nature looking to see if there's a case instead of your input in your best interests. It really is a free country, though. That includes the freedom to make less than optimal choices and live with the consequences.
Now let me broaden the discussion: imagine a man carrying a rifle in his hands, approaching a mall during business hours. Working off the definition, can anyone *not* see RS to stop him and investigate? Bonus: Any time the government has the authority to do something, they have the authority to use reasonable force (as determined by the model "reasonable officer" NOT the more generic "reasonable man") to make it happen. Would such a stop/detention/investigation justify the threatened use of firearms by LE? Why or why not?