I seems strange, but yes, they did keep accountability of small arms even while shoving fully equipped tanks, trucks, and airplanes into the ocean or just walking away and leaving them.
I have heard several stories about supply officers or commanders selling various weapons. I have reasonable cause to believe the "sales" were not, shall we say, quite "kosher" and the officer had no legal authority for the sale. He would simply put the money in his pocket.
Technically, any U.S.-owned item that was not sold under the regulations was stolen, but with millions of U.S. pistols, rifles, etc., sold off after WWII by the U.S. and its allies, it would be impossible to prosecute anyone and no one is going to even try.
Prior to sales of carbines by DCM back in the 1960's, the government did try to prosecute owners of GI carbines for receiving stolen goods, contending that no carbines had ever been released. But then it came to light that Benecia Arsenal had sold ten (10) carbines and didn't record the serial numbers. That blew it, and the U.S. never again attempted to prosecute for possession of an M1 carbine.