And when he called in the background check, it was cleared to transfer?
If stolen, I would think it would've been caught at that point in the process. If not, you and the gun were cleared through a background check. So, you should be in the clear, even if they do come get the gun.
A NICS check focuses on the buyer's eligibility to possess a firearm, not the firearm's status.
It is likely the pawn shop's records were reviewed and a firearm that passed through their possession showed to be stolen in a database. They then produced a record of its disposition and that person then turned the investigators on to the gun's current owner.