FakeTV actually looks like something that could work well as a deterant, as long as thieves don't get too close to the window at which point they'll realize there's no sound coming from the "set", although you could say its possible someone would be listening using a headset in a darkened house late in the evening.
buy a red laser 200mw or greater with continuous light like this:
Play a couple of games of Khet to get a sense of how the beams bounce split and attenuate. Design your grid.
Ideally the walls would be made out of brick or something that could absorb the heat, in case your mirrors slip. If its all glass you have to get it perfect!
Use foam tape to secure khet pyramids and beam splitters temporarily. Then using the lasers that came with Khet and theatrical fog, make fine adjustments. Permamently mount game pieces. Use a black stone as final heat sink for the laser. Buy laser goggles if you don't have any. Test out the red laser outside and get a sense for its burning power and intensity. This can vary even within the same sku! Clean mirrors (and keep clean) on the game pieces! Test with live laser and goggles. Feel for excess heat on mirrors. If you have any concerns about safety at any point, don't proceed with the live laser until you have addressed them. You said fake alarm, but once you did all that you could easily hook up a cds sensor, like the one a garage door, to trigger an alarm...
P.S. I myself prefer fake infra-red curtains, they're way cheaper and safer if not so flashy, though just like the doomsday device in dr strangelove, you have tell someone for them to be effective...
If you build it, they will come. When the bad guys see your red laser security system, they will focus on what it is protecting. Now if you want to use that as a diversion, to get their attention off what is really valuable in your house, then this may work. Anyway, that's my thoughts on it.
I just ran accross this YT video by accident. He has a bunch like this on his channel. More importantly, you can buy cheap lasers from his site, Wicked Lasers (he gives the link in this video). ALso has other sources for laser-related stuff, including simple schematics that you can follow to build your own device.