Smoke detector missing

[Sarah Y.]

If a first time homebuyer purchased a home, finished closing, walkthrough done, inspections done, already moved in, and maybe a month or two later noticed one of rooms was missing a smoke detector and there was no hole in the ceiling like everything was finished & built. Except the builder maybe "forgot" to place a smoke detector?

 My question is who's fault is this, the buyer, inspector, agent, builder...?  & what would be the solution for this?