The weapon wouldn't go back to anyone once it disappears. There are legit and non-legit weapons in the game.
Back in the day we used to teleport steal weapons, armors, and ammo from Hinkley and GM Events (why do you think you can't steal KO players?).
The problem with weapons and armors stolen from Hinkley is that they disappeared when characters despawned after logging out.
Sounds like the two Gauss rifles were "non-legit" weapons, as far as I can tell, and that's why they disappeared. You both logged out with the weapons on your characters, correct?
This is all just a guess on my part.
ETA - Not accusing GM's/Admins, either. Could be a redux of an issue the previous session with a player...