I was rooting around in the back of my car looking for something and I noticed a fleece blanket I keep near my BOB had ice on it.
Sure enough, the 2 16oz stainless steel water bottles in the bag had leaked from the freezing weather. Before you get all "no shyt, Sherlock", I have a few words in my defense. First, before I ever started carrying these type of bottles in my winter BOB I filled them to the top and put them in the freezer to see how they would react. After a couple of days there was some bulging but the bottles maintained their integrity. Second, I have now been carrying these bottles for at least 5 years without any problems. I can only assume that the unusual cold combined with occasional thawing cycles pushed these bottles passed their limits. The plastic threaded caps were pushed out of the stainless steal threads about 1/16 inch. The bottles are similar to this:
The good news is that if I'd had to use the bag for real it would have been OK. The sleeping bag was safe inside its waterproof compression sack hanging off the bottom of the bag. The only things in the BOB which weren't stored in waterproof bags were a balaclava and a pair of fleece gloves I threw in as an afterthought. Once dry these will go into one of the waterproof bags.
The 3 liter hydration bladder is OK, of course, but frozen solid. The purpose of the stainless steel bottles was to have a source of clean ice during cold weather which I could melt on a fire for cooking and drinking. How do you guys carry water in the winter? Do you think the stainless bottles would have been OK if I had only put say a cup of water in them? The bladder is useless in this weather without a long warmup period near a fire, which was the purpose for the bottles.