My friends and me placed a lockbox too. We figured out that only the player who placed it can unlock/lock even if we are in same group. And if you put items with attachments into boxes, bags or trunks they may disappear. I didnt tried to reproduce exactly but it happens some times on the same way. Now we started to detach all including ammo/magazine and it never happens again.
Server: Skaronator.com 0.1.0.3 - EpochMod.com Hosted by Nitrado.net
Epoch build: 0.1.0.3
Arma build: 1.30.127372