Wife has weird ex[perience

Last night my wife was working with the computer. She said that as she sat there at the computer she heard, coming from the floor to ceiling bookcase at her immediate right, a distinct creaking noise such as a door with rusty hinges would made upon opening. As we have had some really creepy issues with this bookcase my wife said aloud, "I don't know who you are, but I do not like people reading over my shoulder". "So cease the spying or I'll inform the DEA". My wife has the suspecian that tunnels in this town are being used by drug runners. Where she gets that notion, I do not know. We are not even certain there are tunnels under this house though several other houses in town definitely do have them. Anyway, she said she heard a short scrapping sound and a "thud" like a door closing. I simply cannot see how much could be going on behind this bookcase unless the walls are a lot thicker than they appear to be. Some of our exterior walls are 4 bricks thick, some may be thicker. Some of our walls appear to have interior metal sheets. As we have never made extensive modifications or repairs on the exterior walls I'm not certain about their interior make-up. But this bookcase has centered on several of our reports of things going ,"bump" that we cannot explain. The rear wall behind the library shelves extend to the basement where they encounter huge granite foundation blocks weighing hundreds of pounds. I'm not certain how a false wall might be constructed to hide a passageway; but there just doesn't seem to me to be enough room to do so. But those old builders could be very crafty; or so I'm told. Or maybe we really do have a spook loose here. Who knows?