The NFPA is somewhat ambitious in writing codes to cover all & sundry....even though very few are actually adopted anywhere. Despite this, they are usually a good source of guidance- you won't often go wrong!
That said, requirements for smoke detectors are typically found in the local building code (for new construction). The UBC, BOCA, and other common building codes all address this issue.
Another source that may be relevant is the local "housing" code, especially in the case of apartments, motels, and the like. Government subsidised housing contracts also typically mandate them.
Lesson: Knowing just the NEC is not enough.
