It's here on the platform, in the locker 137, but you have to pick the lock. Your sniper rifle is here. On expert difficulty there is a first silenced pistol in the game (on normal difficulty you have it from the start and the pickup has a simple 9mm pistol). Also you are given some ammo and (on normal difficulty) night vision goggles. Make sure the two passengers don't see you as you are picking the lock and (more important!) as you are carrying a sniper rifle. It may be easier, if you don't take the rifle at first, but return for it already having a disguise.