When you have no customers, and all reservoirs are full, go the the street meter and check for flow at the meter. There should be none.
If there is, you may have a leak between the meter and the equipment room. I have a $1,200 water bill from last year to prove it can happen.
Next, check the tips you are using on the self serve bays. Make sure they are new. As an example, we use a 1505 tip.
*One big one to check is if you use a water softener (the one that uses salt). See how long and how often it's regenerating. If it does this at night, you'll never notice just how much water can go through one of those babies (until you get a bill).