They might have still had mobile triangulation devices around. They can provide more precise estimates, obviously. Of course, I don't even have any idea if they would work with modern mobile phone technology.

Good point. Same mobile signal-strength gear (with directional antennae) that engineering folks would use. No reason it wouldn't work on a straight RF signal-strength basis, but no idea how well that would (or did) work in the context of the massive, 3-D WTC site. You've prompted me to some research.