They don’t know when they went missing.....I wonder how they account for what is in storage, I’m thinking it’s nothing more than lazy paperwork.
That would certainly be the place to start. I have to assume that the USMC's inventory controls are every bit as tight as the Army's, which I'm more familiar with. Yes explosives do, "just disappear," in training, but even then, there is a paper trail (or more likely these days, an electron trail) that scrupulously tracks where every last ounce is or went.
We maintained a small amount of C4 and certain drugs in our Provost Marshal Office for training our detector dogs, and in my experience, there was no way any could come up short without having a name, date and time for the last time it was signed out, and barring that, there would be a name, date and paperwork for the last dates anybody who "didn't" sign it out would have had access to it.