20+ Gallons an hour leak - I can't find it
Are you on a well or municipal water supply? If on a well, with all of the faucets closed, does your pump turn on? If a municipal supply, same thing, with all of the faucets closed, is your meter running? Is your house built on a slab, basement, or crawl space? Based solely on your initial post, it sounds as if the line supplying your outdoor spigot could be ruptured, whether in the wall or basement, which it seems would be evident enough, in the crawl space, which you should investigate if you have not already done so, or under your slab, which may not be as obvious at first glance, but these are only guesses at this point.