There was a recall on the B+ terminal screw not being tight thereby not allowing the battery to charge. Have you taken it in to get this addressed? hmmm which reminds me that I need to do this as well.
I had this happen once and still have no idea what caused it. It hasn't happened since then, which was last October. I thought maybe the RSE screens were left on as pushing the power button on the console doesn't cut the screens off and not sure if they cut off automatically or not. Either way, hasn't happened since then.
For whatever reason, on both the 2004 & 2006 XC90s, went to start car only to find a dead battery. Both times dealership gave cars a clean bill of health and it only happened the one time to both cars.
O.K. Twice on the 2006 ... left the key in the ON position for two days.
I also had it happen about four months ago. I left the driver door ajar overnight and it killed the battery. I jumped it and it has worked fine since. In my case it had nothing to do with the B+ recall, as I just recently had that done.
We had this happen to our 05 T6, and found out it was because the key fob to the volvo was interferring with the key fob to our Audi and was causing it to activate the kill switch in the alarm.
I think if I remember right I was able to start the car after a 1/2 hour or maybe even an hour later. I was told because of the close proximity of the 2 key fobs, (both on the same key ring) it was causing this. I hope this helps and that nothing serious is making it not start