Just sanity-checking a concept... poor man's self-monitored alarm system
NRF24L01+ for communication. Extreme low power if not sending. Detects packet loss, and automatically resend the packet. 

You would however need a small arduino board at the sensor, to read the sensor, and signal to NRF24L01 to send a message. 

You'll have to put the arduino in to low power sleep mode to get some good battery life. Either have the sensor wake the arduino up with an interrupt, of poll the sensor like every half second, and immediately go back to sleep.