We know nk2 file in Outlook 2007, it is an auto-complete file which stores contacts, when we type a new message. This is an important file to most people, most of the people not aware this file until it is gone.

Outlook 2010, Outlook 2013 & Outlook 2016 do not use nk2 file. They use

Stream_Autocomplete_0_[long GUID].dat

file to store the contacts.