This app was first announced in February 2012 and then released in April 2012: we discussed it at http://pistehors.com/news/forums/viewthread/1174/#4219.
We were unsure about how good the relative accuracy of GPS would turn out to be. The relevant page of the web site http://www.isis-application.com/en/internet-and-bluetooth-search-scan/ says
“within the last few metres of the casualty –within the imprecise GPS zone- the iSis app fine combs the area thanks to Bluetooth” and shows a diagram of a grid search.
I think this means that the final stage of the search in this app uses Bluetooth signal strength maximisation, in much the same way as in an analogue transceiver. Bluetooth works at a wavelength of 1.2m, so the final search will progress from far field towards near field. This will make it a bit more complicated and rather less sensitive than a transceiver, which always works in the near field.
I agree that it would be very interesting to hear from someone who has tried it on the snow.
It would also be interesting to know more about the signal processing, particularly the filtering, and how far it has been optimised. Our discussion last year made me feel that it should be possible to do better than a few metres using GPS.