Good observation! Unfortunately that door leads to the pipe riser. The stains are from years of water leaks (this building has not been well maintained). The door pictured is the only access point to the room, so breaking into it doesn't get you any closer to an exit.
You're death here provides slightly more information on the situation to anyone else playing along. You get a nod of approval from death as he collects your soul.
Edit - point of interest.... had the stain been from smoke damage, there would also have been stains above the door frame.
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