Cold is the real killer in my experience. I had a spare lithium battery in an unheated garage for a winter and it lost most of its charge sitting there. Brought it inside to room temp and it held fine after that. Pressure changes might play a small part but temp swings are usually what drains them. A sealed case helps because it keeps the temp steady, not just the water out. If your case has a gasket and stays shut, that's probably doing more for you than you realize.

Wet towel in the same sack was your real problem, not the matches.

Man, three weeks is nothing, sounds more like the post moved than the brace.