I spent 5 years as a Police Officer in Oklahoma. I can only contribute based on Oklahoma Law.
The Probable Cause to stop was, as Airscrew said, speeding. Once the traffic stop was complete, the officer more than likely asked if the young man had anything illegal in the vehicle. This could have been triggered by several key indicators like nervousness, failure to make or maintain eye contact while being spoken to or, like Airscrew put it, they saw where he was from and had a sense of suspicion due to an obvious drug trafficking problem on that particular road. I say obvious because Airscrew knows about it. (Unless, he's in on it

) Once the officer suspected and listened to his hunch, he most likely asked for permission to search. Here is the "inside" information on this question and why cops ask it. If I ask you, "Do you have anything Illegal in your vehicle that I should know about?", your involuntary first response if you truly don't is, "No". Now, if I ask that question and you say, "Not that I know of" or similar, other than "No", then I will be asking for consent to search your vehicle. You bet, 100%, that this kid could have denied permission to search. My opinion is that this kid either didn't know the contraband was there or knew it was there but didn't think it was illegal. He probably gave a trigger response that cops are trained to listen for or gave a nervous sign and thats when they asked for consent.
I'm with the majority here when I say, "Brass Knuckles?, Who cares?". I don't understand why the kid got locked up for that other than Barney Fife needed something to kill his shift with. But I don't think the Probable Cause debate and subsequent call for attorneys and such is necessary until all the facts are in.

Some of you need to take of the Tin-Foil hats and let your brain breathe.
Hope it turns out okay Airscrew. Glad the boy is okay.
