Devices will never replace the impact of in-person engagement.

All contracts, options, or other devices not based upon a substantial consideration, or that are otherwise employed to permit an unlicensed person to sell, lease, or let real estate, the beneficial title to which has not, in good faith, passed to such party for a substantial consideration, are hereby declared void and ineffective in all cases, suits, or proceedings had or taken under this chapter; however, this section shall not apply to irrevocable gifts, to unconditional contracts to purchase, or to options based upon a substantial consideration actually paid and not subject to any agreements to return or right of return reserved.

Every year there are more and more electrical devices that draw power.

I bet you can google a sensor or device that would provide this early warning system.

- No other security devices as of yet.

I didn't know code anywhere allowed more than one device to vent to a flue.

The definition of a dwelling unit under 280A(f)(1): The term “dwelling unit” includes a house, apartment, condominium, mobile home, boat, or similar property, and all structures or other property appurtenant to such dwelling unit.Merriam-Webster: Definition of appurtenant 2: Auxiliary, AccessoryDefinition of accessory 1a: an object or device that is not essential in itself but adds to the beauty, convenience, or effectiveness of something elseDeeds don't matter. 

I think the most important takeaway after follow local and state ordinances would be to ensure that the devices are maintained and replaced when needed. 

A couple laminated sheets in a nice binder near the TV include instructions for devices, garbage pickup day, policies, etc.

The best device I have found is the EMon DMon (and no I don't have any affiliation with the company).