Its happend twice in a row, which is 100% of occurences.
First time, I had a 75 round drum on my akm, but did not have 2x2 slots in inventory. I wanted to close a door, but accidentally it hit reload. So it put a 30 round mag in the gun, and the 75 round drum dissapeared. searched, with 2 friends, for several minutes. It was not in the current room which I was in the center of (second floor of police station), nor was it on the ground floor or roof, or in any of the adjacent rooms.

Another time I had an SKS in my hands, and a AKM on my back with no mag. I bandaged myself after getting hit by zombies, which auto-dropped the SKS. However ,it was completely gone. I was outside on the runway at the time. I waited several minutes, and searched all around, also checking "vicinity" in inventory incaswe it was obscured or similar.