Complex sentence: Interrogation of Neumann reveals that Mirus had been wandering off into the tunnels for several weeks and treats the tunnels as his own personal property, as if something is down there that Mirus didn't want him to know about.
Split into simpler sentences: Interrogation of Neumann reveals that Mirus had been wandering off into the tunnels for several weeks. Neumann also reveals that Mirus treats the tunnels as his own personal property, and that it was as if there was something down there that Mirus didn't want Neumann to know about.