If the arrow did go in 6" could be a high single lung hit no blood trail. If the buck was pushed at all after the hit he could go a really long ways looking for either water or security before expiring. I hope you locate him quickly. If you do find him dead great caution must be made about consuming any meat. Personally, I would tag him out and discard the meat just to be on the safe side, unless you find him very warm and not bloated. Sorry for the graphics, just a cautionary note on meat safety.