As someone who has been responsible for property disposition (though much, much lower in cost) within the Army, I can entirely see how this happened.

Let's say you report something as broken. For whatever reason, it cannot be repaired. You are keeping it on your books until you get your replacement. The replacement comes in, and you now have the task of turning the old one in to be destroyed/decommissioned. You assume (or are told) the new one has been already added to the property book. However, the new one hasn't. For a while, the old one is retained on the property book until the new one is in the system. Whenever you do your property count (by serial number), it is noted the new one is the replacement for the old one, with the turn-in paperwork for the old one maintained as proof.

Suddenly the old one is off the books, new one isn't on it. Huh, that's odd... So you go to the records office and get the new serial number added to the system. You're done, right? Not necessarily. Just because locally it is fixed, doesn't mean it is in the centralized database.

Accountability of high value items is a pain, and in my case it was left to someone that had 2 years in the army and took a 40-hour class in security and record keeping.