I had a mosin on my back and an AKM in my hands I was shooting at a zombie and my AKM disappeared. I retraced my steps and I found it next to a glowstick I had previously cracked. The rounds I fired were missing from the magazine.

I had used the context menu to crack the glowstick on the ground and it played the cracking animation without holstering my rifle. I guess this drops the weapon without informing the client (or server).