When your job means handling weapons all day, forgetting about a set is no more difficult than a tech worker forgetting about a pile of blade servers. It happens. Militaries implement systems for dealing with such things. For instance, red amnesty boxes are common at canadian bases. A rocket launcher may seem scary and horribly dangerous, but sleep beside them for a few months and they become very normal.

I'm pretty sure weapons go missing all the time.

Weapons do go missing all the time. Or at least weapons with the same serial/registration numbers of destroyed weapons show up all the time, be it at crime scenes, terrorist orgs, or weapons collectors.