phantom leak that fixed itself?
Luck, most likely. 

You probably have a slow leak, which you could treat by replacing the tube or just keep pumping air in as necessary. Less plausibly, theres something up with your valve. You could try taking the tube out of the tire and pumping it up with a bit of air and putting it in your bath tub to see if you can find some bubbles. 

[In the comments, DRH has the reverse ordering on plausibilities]