I have an electrical box in the ceiling, currently with all the wires capped and a blank faceplate over it. Two cables coming in, both with four conductors (black, white, red, and bare).

I don't know whether the red is meant to be a traveler for a three-way switch, or an interconnect for smoke alarms. The weird part is that this particular breaker already has both: all the smoke alarms in the house plus a hallway light controlled by two switches. (Other three-way switches in the house use a red wire as the traveler.)

Is there a good way to tell which it is? I'd like to put a smoke alarm there, but obviously I can only do that if this isn't controlled by a switch and interconnects with the other alarms.

The house was built in 2002, if that helps.