Leaving lost neutral wire
Chase down the matching neutral and remove it. 

Otherwise it will drive you crazy, and be a loose end of wire unaccounted for.  What's more, it could wind up still being in use by some other circuit which shouldn't be there.   Part of the reason to fully explore an electrical system is to look for surprises.  

Besides, since all wires must terminate in junction boxes, it will violate code and fail inspection.  

I don't agree with my colleague that it's necessary to tear apart the ducting system, I feel unreachability is a defense.  But to play that defense, you must positively identify both wires of that circuit, both where they enter and exit the ducting, and fully remove all of that.