Earlier quoted context omitted.
"Registered to a different person" -- for the most part, guns are not registered in the USA. Was this in a state or city with a registry for AR-15s? "Loaded straight into fairly skinny jeans without checking the safety." -- Glocks do not have safeties. The idea with most newer striker-fired pistols, like the Glock, is that the gun goes in a hard-sided (molded nylon or kydex) holster. If it's not in the holster, it's…
> "Registered to a different person" -- for the most part, guns are not registered in the USA. Was this in a state or city with a registry for AR-15s? Handguns are registered to a tiny degree. The NICS checks records may be kept only on paper, and are kept by the gun dealer until they close business. Then the BATFE or a related govt. office gets them. By order of congress, this office is not allowed to computerize.
A registry connects a particular gun -- a particular serial number -- to a particular person. A NICS check doesn't do that -- it's a criminal background check. Just because you have a NICS check, it doesn't mean you bought any particular gun. You could pass the background check and then decide you don't want the gun after all.
You may be referring to a Form 4473 -- the DRoS (Dealer Record of Sale) -- which does record which person received which gun (with serial number) -- and all licensed dealers maintain these records, for all guns they sell (not just handguns). These records form a sort of a search tree, since the manufacturer records which wholesaler the serial number went to the, the wholesaler records the retailer and the retailer records the end user. These records are useful in regulating the traffic in arms; but the decentralized nature of them means that the government can not go on fishing expeditions -- can not just go out and query the database and see what looks interesting today.
Here's how the DRoS are used in regulating the arms trade. Say police come across a big box of guns from Colt. They can go to Colt with a suitable warrant and say, hey, where did these go to? And then go to the wholesaler, on down the tree. It turns out this information is often sufficient to localize shady gun dealers who are trying to sell to the mob, the cartels, &c.