The case is locked using a combination lock. The sound it makes suggests that it contains several items. An entry on the top side of the case made with a marker indicates the owner's Duty call sign.
― In-game description
The Duty member's case is a mission item in S.T.A.L.K.E.R.: Clear Sky.
Description[]
The Duty member's case is a mission item for a Duty squad leader in the Agroprom, Return Duty member's case. It is located in a chest behind the Duty squad in the underground area the player cleared for Duty as part of the main mission line.
Re-enter through the tunnel at the top of the hill. At the bottom of the shaft, go straight and it is in the chest behind where the Duty squad is sitting around the fire.