I already have a secret room in my house that is well hidden. Not because I put it there, but because I found it after I moved in. the sliding door into the room blends in with the wall so well that you can't tell there is anything there. Also it can lock from the inside with no way to open it from the outside(that I have found). I currently have some of my random junk and a Tv + DVD player for times when I just feel like not being screwed with. So, to anyone that has the capability, add a secret room somewhere, or make a currently existing room into a secret room(it's really easy to do, just remove the door frame, door handle, etc. Paint over it to match the walls. If you have a hinged door, hinges on the inside. make sure there are no obvious seems, or cover it up with decorative things[bookcase for example]).
Side note: When I first found the room I was expecting to find a dead body, luckily that didn't happen.