Remember it's not just no mode 4 response and NCTR print, it's any two sources that don't give a response, i.e. 2 F-18 mode 4 interrogates, 1 F-18 and awacs, 1 F-18 IFF and 1 F-16 NCTR, etc... There's also the possibility of someone PLID'ing a track file as hostile or unknown.