This is another of my most popular Youtube channel videos. I made this one about 5 years ago (2008) and HowCast channel ripped me off. Granted, their version has a much more professional production value, so I guess I'm at least flattered by it. But, this is the original!
Anyway, the idea is to make a hidden compartment in your wall that, upon first glance, just looks like an electric outlet. I know, if someone tries to plug into it, it won't work, but I doubt someone would be inclined to rip it out of the wall--they'd just move on to the next outlet.
Notice the miniDV tape? Yep, what spy would be trying to conceal one of those nowadays?