Nest Smoke Detectors – How to Position Google Nest Smoke Alarm in a Room

All three wires get capped separately with wire nuts. You need to cap the neutral (white) wire because it can carry current from another part of the circuit. Wrapping the nut in electrical tape is a good idea too. They make nuts that are good for holding a single 12-gauge or 14-gauge wire (the ones I have are dark blue).

You don't need to do anything special with the ground wire, just be careful when you're pushing the other wires up into the box that you don't accidentally touch it to bare copper on one of the other wires.

Also remember that electrical junctions have to be accessible. In your case, this would probably mean covering the hole in the ceiling with a blank face plate and installing the battery-powered detector somewhere else: I don't think you can use the base plate for the new detector as the cover. I would also be concerned about drafts through that hole reducing the effectiveness of the detector.

According to the guide handed out by my local permitting office, which doesn't cite electrical or fire codes but does detail what they look for:

A smoke detector should be placed inside the door of each bedroom 24 inches from the nearest walls or centered on the doorway 24 inches from the wall holding the door.

A combination smoke alarm / carbon monoxide detector should be placed in any hallway that leads to bedrooms, where it enters the main part of the house, again, centered in the hallway on the ceiling and 24 inches from where the hallway connects to the rest of the house.

All of the smoke detectors should be hardwired to a circuit on the house's mains and should talk to one another via a third wire.