Popping the case open reveals the spartan interior:

Not much to mention here, we have a PIR sensor, a PIR dedicated chip, and a radio module.

The PIR chip is an Elmos E931.96 IC with two signals: Interrupt/Serial Out and Serial In. 

So far this looks extremely hackable. I could easily replace the module with my own header, plug this into an arduino or raspberry pi and call it a day. I don't think that's enough though, I want to see if I can break into the module.