Why is water being consumed when my shutoff valve is closed?
It's clearly NOT the shutoff valve - water is going somewhere, and the valve is shutting it off. So it's doing it's job. There has to be a pipe, or a branch off a pipe that is going somewhere and running water, after that valve, from what you have described.

Typically where water is heard and the water meter is spinning but you don't see any, it's going into a drain somewhere - why anything tied to the "outside faucet shutoff under the house" would do that, I surely don't know, unless it is irrigation or pool related - my usual suspect inside the house (because I had one that did this occasionally, and when it kept doing it every so often after I cleaned the valves to stop it each time, finally replaced it) is a water softener, but that seems highly unlikely here. 

Depending strongly on the pipe layout, you might get an indication of which branch is most suspect by measuring the pressure at each outside faucet. Not guaranteed, but possibly one will be considerably lower than the others, and that would be the one to double-check the pipe to, for any branches you may have missed.

Toilets are the other usual one, but they tend to be more obvious as far as being seen/heard misbehaving. Not to mention that you'd notice it not filling with the shutoff closed.