In the German forces, nobody gets off until the unit armory is counted and safe. When I was in deputy CO, it was the most important thing to ask in the late afternoon: “armory closed, counted and safe?” So I can imagine, the offender took the gun over the day, made it somehow off the base and kidnapped his ex.
Yeah, it’s the same in the U.S. Army, too. He either took it during the day, like you say, or the armorer didn’t actually do a barrel count but said he did.