Originally Posted by
MudPie
Perhaps you had a thorn or piece of glass embedded itself into the tire and through the tube. The object remained in the tire, and essentially acted as a plug while in the tube yet allowing air to escape slowly. Then, eventually the object worked its way out of the tire and allowed the air to escape thru the unplugged hole.
Since all of this is pure speculation since the OP didn't take the time to inspect his tire to figure out what happened, I am going to agree that this particular pure speculation is the most likely. Especially since the OP reported hearing the air leaving the tire. The idea of a pinch flat due to a soft tire is appealing pure speculation as well, but normally you can't hear the air coming out from a pinch flat.