Valves usually turn right to close. The Player sometimes has to turn valves left.
NPCs do not usually react to lights being turned off.
A tactic used for stealth is to hide a body. However, the Player is not required to hide whatever weapon the person he neutralized was carrying, which would not make a lot of sense, stealthwise, as other characters would wonder why a weapon is lying upon the floor.
Only rifles are DNA coded; no other weapons are.