There are a lot of times when we take theft of property reports when there is actually no proof that someone stole it. It's done more of a courtesy and giving the citizen the benefit of the doubt. While I can't condone the officer getting drunk AND possessing a weapon (no matter what it was kept in) I also don't see what harm it does to give him the same benefit of the doubt we'd give a civy. Of course, there's very little information available to make a firm decision on this either way.
He had the gun in the bag, he had the bag in an area where it's theft / loss was possible, the gun is now missing and he didn't "misplace it".
While I hate to quote ....
"... when you have eliminated the impossible, that which remains, however improbable, must be the truth." Sir Arthur Conan Doyle.
(Before anybody brings up that this quote was used in a Star Trek movie, please don't)