With a creak a tad too loud, the door slowly swings inwards. As he pushes, he curses silently at the tweaking door hinges. He steps inside. The shack is barely the size of an average school classroom, it looks as if it has been abandoned since the beginning of time. The smell of long-rotting food fills the air, fungi has decimated the plastering on the walls, age-old dust forms a carpet-like layer over the splintered floorboards. Past the doorway is a narrow, short corridor which ends in front of a door. He reaches into his jacket pocket, feeling for the Sig-Sauer P228 Semi-Automatic Handgun. Clasping his right hand around it, he pulls it out into the open. If his suspicions are by any chance correct, he would be needing it a lot in the in the next few minutes. He pauses, holding his breath still. A look at his wrist watch. 1700hrs. Sunlight beams through the open door which is the only opening in the structure so far. He treads forward, the floorboards creak menacingly, there are pieces of decaying animal dung where the floorboards are splintered, producing a stomach-heaving stench. The Sig stretched out at arms length in front of him, he reaches for the door at the end of the corridor. Tries the lock. Locked. He reaches into his jacket pocket, pulls out a tiny piece of curved steel wiring about three inches long, inserts it into the lock. He wraps the wire around the pins holding the lock, prying them out one by one. It takes barely a few seconds and the lock gives away. Stowing the lock-pick, he clasps his right hand firmly around the Sig, his index finger on the trigger. A deep breath. He takes a step back, raising his right leg........ With a loud thud, the door crashes to the ground due to the rusted, weak hinges giving way, spreading a cloud of dust around the room. The Sig still in his outsretched arms, he steps inside and takes a look around. He immediately takes note of the sightings. An empty wardrobe to his right, walls plastered with fungi, a rusty cooking stove in the left corner, in the middle of the room lies a torn mattress on top of a piece of wood which isn't exactly fit to be called a bed. In the right hand corner lies a dust-laden desk with a few drawers beneath. By far the most exasperating, is the open window at the other end of the room. He curses silently to himself. He reaches into his pocket and retrieves a disposable cell phone and dials an eight-digit number. "Go for Mitch." A dark, heavy voiced answers on the other end. He gets straight to the point. "He's not here, we were too late." The words came so casually, and he can sense the disappointment on the other end of the line. "I think they got to him first, or he had the drop on us." "Shit." "I'm gonna take a look around, see what he may have left behind." "The Department's so not gonna like this, we spent three years getting this far, now we'll spend the next three starting from scratch. Fuck!!!" Mitch is fuming. He can feel it. "I have to go now, Razor out." He snaps the cell phone shut, moves towards the drawers.
Your writing style is more attuned to screenplay than prose. Either way, I suggest omitting -ly adverbs in general. Use your arsenal of verbs instead of throwing in modifiers.