All off map units live at -1,-1. So the missile will arrive from about there. (Offmap arty arrival is fixed by player side, so does not originate from the 0,0 hex. The missile firing offmap is a bug, and so has no correction applied)
This was the reason you got the "amusing" bug in the original SSI code with retreated helicopters fighting crews or batteries offmap - thier original code had no offmap test in the targeting routines for direct fire
Looks like there is some of this legacy "no-testing for offmap" with the SAM code. I'll hunt it down after the SPWW2 coding is done.
Cheers
Andy