Some guy managed to open locked doors - Hack?

Hi,

i encountered a guy from CZ region on my server that were able to open any locked door from any player. I received tons of complaints this morning, even his friend shouted (after we banned him) "MY FRIEND WILL KEEP ON DOOR HACKING THE WHOLE SERVER"

I checked my code and this is the only hook I'm using:

Code:

        private object CanUseLockedEntity(BasePlayer Player, BaseLock @Lock)
        {
            if(Utils.GetAdminLevel(Player) >= Utils.AdminLevel.Moderator && Invisible.Contains(Player) && Player.IsFlying)
                return true;
            return null;
        }

Anyone else encountered this issue?
I'm trying to fix this via a plugin atm.