And here's a follow-up question.
As soon as I hit send on this thread, I hear a loud blast and watch one of my tires go flat. I had ridden the bike about 40 miles this morning with no problems and it's been sitting in my living room for the past eight or nine hours. I remember learning about spontaneous combustion about fifty years ago in science class but spontaneous bike flats??? What happened?
Karma!
Seriously, I've had the same happen to me. I had replaced the tires and tubes on a bike I had bought used, and put a few hundred miles on it. I propped it up to take pictures, went inside the house and came back out to find it laying on its side, the front tire blown. The tube had given up along a seam in my case, but the other explanations above are feasible as well for yours.
I'm just glad yours gave out in the living room rather than on the road!
For the original question, yeah, CO2 leaks out quickly. This is yet another reason to check your tire pressure prior to each ride.