The Halperin Hotel is one of the early districts you get to explore in Dead Island 2. Like other areas in the game, this once-exotic locale is now home to flesh-eating zombies.
Halperine Hotel was the evacuation point for the people of Bel-Air and Beverly Hills, which is why you might come across several military vehicles during your visit. Interestingly, one such vehicle has a locked supply case. Here's where you can find the Military Supplies Case key in Dead Island 2.
Where to find the Military Supplies Case key in Dead Island 2
You can find the Military Supplies Case key on a Crusher called Drill Sergeant. This named zombie patrols the streets that lead up to the Halperin Hotel. However, he won't spawn here until you have finished the main quest, "Room Service For Major Booker."
Come back anytime after finishing the aforementioned quest, and Drill Sergeant should spawn. You can find his exact location in the screenshot below.
Since he is a Crusher, it might take more than a few hits to take down the Drill Sergeant. Once you have taken him down, loot the Military Supplies Case key off his corpse and head towards the zone exit that leads towards Bel-Air.
Instead of turning right to go Bel-Air, keep heading straight, and you will come across the military vehicle. From here you can unlock the case to get the Superior Guarding Military Knife.