Maybe I’m having a bad day, but what does the first sentence have to do with the rest of the OP? How can you be jailed for an outstanding warrant if the charges have been dropped?
I read it as, he had an outstanding warrant in error. Basically it sounds like he was charged with a crime, and got the crime dismissed or adjudicated in some way through a court hearing. But somehow a record keeping error resulted in a warrant being issued for his arrest. It’s possible maybe somehow wires got crossed and it was never correctly communicated that he’d dealt with the issue in court, and somehow a system flagged him as being a “failure to appear” or something and that got a warrant issued.
Anyway, I don’t know how it can happen but I’ve heard of people having warrants out for them for issues that had already been resolved. Just a matter of the left hand not knowing what the right hand is doing in a large legal system.
OK, I get that. So, was he taken in on the mistaken warrant and then issued a ticket for driving without a license? Or are the two issues not related?
I would guess they stopped him on the warrant, most likely they ran his plates and got a warrant notification. Many do not realize that, but many police officers will run plates through their system throughout their patrol shift. They do not need probable cause to run your plates, but if they get back a notification that the owner of that vehicle has an outstanding warrant they can pull you over for it. Then, when during the course of the traffic stop they ask for your ID and you fail to produce it you would ostensibly be running afoul of the laws mentioned in this thread that require you produce ID on demand while driving.
It’s also possible the OP left some details out. Maybe he was originally stopped on an ordinary traffic violation like speeding or running a red light, and as the officer ran his information the warning came up.