Same thing happened to me today - a jet at least 400nm outside of my airspace and not known to me at all called me up requesting Wx diversion as the comms equipment for his responsible sector were playing up in the weather. As I was trying to figure out who to coordinate with and tee up the clearance (as I did not have a direct line), again I was "told" by the aircraft (Within about 1 min) that they were diverting without the clearance.
Agree with Neville - any info would be appreciated!
