Weird sequence of events

I'm told this is the better forum for this post, so moving it here from General.

07; 200k mi; After being shut off for about 20 minutes, car was completely dead; had all the symptoms of a dead 12v battery - only a couple dash lights which were very dim, plus very strong rotten egg smell. Occasionally other dash symbols would light up or flicker, but still wouldn't respond to the On button. Even though I didn't think they were turned on, the four-ways were flashing, but they were very dim - it wouldn't have been noticeable in the daylight. I pushed the four-way button on the dash to turn them off, but they continued to flash. Went to sit on a nearby bench to call AAA. Phone tried to pair with my in-dash which surprised me with a dead battery. Looked over at the car and the dash was lit up pretty normally, and the four-ways were at full brightness. Got back in the car and everything was more or less back to normal. The 12v had definitely died momentarily because the clock and other electronics had reset. The smell of rotten eggs was really strong at this point. Car started up and we drove on back home normally.

I'm not the original owner and don't know if the 12v has ever been replaced so I'll have it tested and probably replaced tomorrow.

But I wanted to hear if any of the other odd symptoms were noteworthy, especially the smell.

New information:

I just finished running the self diagnostics. The unloaded voltage was 12.4; I put the car into accessory mode and the voltage dropped to 11.9 where it remained. I increased the load on the battery by lowering the A\C temperature setting, and the voltage dropped by a reasonable amount for each increase in load, but then remained stable. Finally I turned the car on and the voltage went up to 14.1. So the diagnostics show no evidence of trouble with the battery or the charging system.

Any thoughts?