Any adjacent rooms or apartments? Any ductwork that could be carrying the sound?
Any recent work, construction, or repairs that might have covered over some device?
Double check all electronic devices - even those with external power connections.
Look in boxes and drawers, under seat cushions, and especially in the crevices between seat cushions and chairs/couches.
Wait until very dark and quiet. Sit in the area and listen. May help you focus on the location of the beep. Move in that direction - listen again.