When Nest Protect came out I was really pleased because Securifi had all the sensors those had displayed and able to be used for Rules. Then Nest changed their policy and restricted companies from doing that... So that was a bummer.
The Halo was another, pretty awesome sounding, one. Multiple sensors plus a controllable RGB light ring. Looked decent, but support never really got added so I was never able to use it fully. Really too bad they went out of business. If full support had been added I would buy up the remainder of them locally and be happy for the years until they need to be replaced.
If you find something interesting, let us know. A smoke detector is usually in a good spot for motion, temperature, humidity, and air quality sensing. All those things can help for a smart detector in the first place.