In a serious case of out-of-the-box thinking, a design team at Roost in Sunnyvale California have come up with a method to add Wi-Fi connectivity to your old smoke or carbon monoxide alarm by just swapping batteries.
Your smoke detector uses a 9 V PP3-type battery right? The new Roost battery contains 9 V lithium cells together with a processor and Wi-Fi chip in the standard PP3-sized battery outline. It monitors the battery voltage and power drain to alert you via Wi-Fi and a smartphone app when the alarm has been triggered, even if you are not at home.
For more detail: New Battery Smarts-up your Smoke Detector