Empty Tank?

My kegerator setup is outside in my detached garage. I live in Wisconsin, so, now that it is winter, I am unable to keg because it will freeze out there. I left my empty kegs and my 5# CO2 tank in the fridge out there.

Well I usually don't secondary but I'm doing a dry hop recipe so I decided I would purge my better bottle with CO2 before I racked the beer into it. I went outside and grabbed the CO2 tank and brought it up here and it was basically empty. I racked into the better bottle and then at least had enough CO2 to purge the headspace.

But I'm confused what happened. I had the CO2 tank filled pretty recently so clearly there was some sort of leak involved. I want to say that the valve on the tank itself was closed, but that's probably not true-- I didn't really pay attention when I used it. When I put it in there for the winter, I double checked just to make sure it wasn't releasing pressure and it wasn't. Anybody have any ideas what happened here? I'm not even sure it would be the cold weather-- it's warmer in that garage than outside and it seems that I left a bottle of beer in there which didn't explode. So it couldn't have gotten THAT cold in there...

I'm just surprised that it started leaking for no reason.

What is the freezing temperature for carbon dioxide?

around -109.3 degrees Fahrenheit (-78.5 degrees C).

Don't think it got that cold.