The simpler solution is to just remotely set off an alarm. I do this frequently with the "Where is my Android" thing. Internet-enabled devices should include this capability by default, anything more is just over-engineered.
I imagine they could be somewhat close together (e.g. same room), which makes setup way easy.
I can't imagine it would be very difficult to make, provided small Wi-Fi capable devices are sufficient, it just needs some work on the details.