Connect open neutral in box?
I have seen two 14/2 cables used instead of a 14/3 cable. 14/2 is a hot (black), neutral(white) and a ground(bare copper), 14/3 is hot, neutral, and a load (red). It is used for three-way switches or smoke alarms where the third one is used to signal all alarms to sound when only one detects smoke. 

I don't know what your setup is but it may be similar to the smoke alarms, which would mean that the neutral is not used. The original installer may not have had 14/3 for whatever reason.