Prototype of the SSP-100M protective suit. There's data that it was in the Zone before the Second Explosion though, because there are many artifacts integrated. For example, the outer layer of the suit is covered in Pellicles. - Inventory description
These tend to be in short supply, but an early tortuous route through the game working for Alex and Milo could arm the player with one; or the coordinates of Anomalous materials (Forgotten) might become available in the process (possible, but unlikely)
Jump the gantry.
Wear the first SSP-100M and climb to the top of the gantry, then jump onto the control room roof and climb through the window.
Pick up the loot and jump back out of the window.
This is much easier and quicker than the first exploit, but the player should provide for impact and bleed control when making such intentional drops
Obviously the 'door' remains locked until the player returns with the key (if ever)