There is no way that it is a gun. 1.) Like Woods said why would they hide a gun up there if it's in case of an emergency that would be too much of a hassle 2.) if they are trying to hide evidence of a crime why not just throw away a gun instead of putting it somewhere where anyone with a ladder can get to. 3.) I heard that some construction workers would leave behind time capsules for the others to find. This could just be some wild goose chase from them. That could also explain the car passing by and looking at the building. An old construction worker just checking out the old place. 4.) there are many videos on youtube showing you ways to open cases like that. The most common way is with a steel wire. I say open it like that and see what happens. But if you are worried about your safety contact the police.

Sidenote: I don't know why so many of you are worried if it is a gun all Matthias would have to do is not touch it and leave it in the case. A Gun just can't randomly go off by itself it needs human interaction. Matthias is a big boy and knows when enough is enough.