I'm sure that the sand loves that water so it wouldn't stay long.
But think about how much water we are discussing.
You jump from 250 gallons to 11,000.
Now, I've never been to that area so I may have some wrong ideas.
But if 11,000 gallons a day went out wouldn't there be some very healthy and happy growth out there somewhere?
And if it did go out in one day then that area would remain wet for at least that day.
You definitely have a strange problem.
What about the meter? Is it manually read?
Could your tenant take daily readings for a week or two and see if they match what the water company says?
Think of this: if you have a 3/4" meter, which is common, and if you run that into a 1" sch 40 pipe then max flow is around 13gpm. 11,000 /13 = 846 minutes = 14 hours of run time. And that is the water just flows straight out, not through any heads or drip lines. Something doesn't match.
Other grasping at straws questions: any work in your neighborhood, particularly near your house, that could explain?
Any chance a neighbor is 'borrowing' water? I found a system a couple of months ago where one zone was tied into the neighbors sprinkler system and using their water.
Sure am curious as to what you find.