This has happened to me on my '15 volt maybe 3 times in the past 6 months or so. When it has happened, parking and turning the car off doesn't give the usual "whoosh" shutdown, but rather an abrupt, everything is off. No power locks, no trunk, nada... Then after maybe 10-15 minutes the car springs back to life. It's almost as if it was doing a reboot.
One after symptom I have noticed is that when it comes back to life, the compass is no longer calibrated and I have to pull a fuse to reset it. Everything I have read here suggests either a wonky 12v battery or a bad connection to the 12v battery. In my case, I have a subwoofer and amp connected to my 12v battery and I've been meaning to unhook and clean the connections but have just been too lazy to be honest.