During the base infiltration, sneak through the sewer and reach the hangar ahead. Take out the two soldiers there then sneak through another sewage tunnel to reach the next hangar with a truck and the base commander who will be marked as an optional target. If you want to maintain stealth, let the commander walk out then grab the collectible on the crate behind the sitting soldier behind the truck.
This is found inside one of the barracks near the radio tower. This can be retrieved while sneaking to kill the base commander (who will circle around the immediate area) without alerting anyone or during the actual firefight after providing overwatch support to your allies.
Near the end of the mission, you’ll have the updated objective to climb the tower overlooking the airfield to provider overwatch again to Pierson’s assault. As soon as you enter the tower, get this memento from the crate to your immediate right.