Hi,
I had a similar problem with my 992 Carrera S Cabrio, not long after I took delivery (it was an OPC Demo) I went on a run over Snowdonia and travelling through the mountains, the SOS error message came on. When we arrived at our destination, I tried switching the car on and off, locking it several times, but still the message was there. During the holiday it remained on until, I returned back accross the mountains and into Shropshire when miraculously it extinguished. I never had any problem during the following months...... until I returned to Snowdonia when exactly the same thing happened. However on this second occasion, the message remained even when we had returned home.
I booked the car in with the selling OPC who carried out a software upgrade which seemed to change some aspects of the dash layout slightly, but after that the error didnt return although on one occasion travelling on similar mountain roads, without any reason to, the SOS system dialled up the emergancy services, I explained it looked like a fault but it never happened again.
I can only assume it happens when the SOS system looses the satellite signal. Perhaps worth a call to your OPC. Please keep us informed with any updates.
Regards David