The suggestion that they used cellphone trialgulation is pronbably correct.
There are a couple of now proven systems out there that can find you if your phone is simply switched on. They were originally designed to find people [recently] buried under snow but it became clear that that they act like an locator beacon for many other scenarios including ones where the missing person is out of cellphone range and SAR aircraft are now carrying the systems. Triagulation is simply another step in the same direction.