Are you familiar with CO2 fire extinguishers? When the CO2 leaves the extinguisher it gets super cold. If your tank is leaking through the beer keg it is taking some of the heat away when it escapes. It sounds like it wouldnt take a lot to get it down to freezing. This only happens when you are away?
Is the amount of beer the same as when you left?
Do you have teenagers?
...sorry - had to throw that in there.