On Christmas morning one of my toilets broke (little plastic piece holding the float). This turned out to be a good thing--silver lining and all, because I noticed that over the span of a few hours the tank would quickly approach empty with the intake turned off. My usage had been creeping up over the past four to six months, and it seems that this toilet using up something like 20 gallons or more per day when not even in use was the culprit. So, I'm expecting a noticeably reduced amount of usage in my next reading now that it's fixed.
Moral of the story: maybe turn off your toilets once in a while to ensure they're not doing something they shouldn't be.
"...but ya doesn't hasta call me Johnson!"