The electrician ran 14/3 to a metal box. There is no grounding post on the Kiddie i12010s smoke detector. Can I just cap the ground and call it done? Doesn’t seem right but I’m not sure how to proceed.
That looks like a plastic box to me. If it is metal, code requires that the box be grounded. If all the external parts of the smoke detector are plastic then a ground is not required (the same is true with light fixtures).
I don’t usually cap bare grounds. I just tuck them in the box (the entire length is conductive, so I don’t see the point). If it makes you feel better, put a cap on it and tuck it in the box.