Although they could see nothing wrong, they had everything checked for leaks or problems: first the water meter, then outdoor pipes, indoor pipes, underground pipes, faucets, toilets, washer, ice maker, etc. -- all to no avail. 

 One day Jim was sick and stayed home in bed, but kept hearing water running downstairs.   He finally tore himself from his sick bed to investigate, and  stumbled onto the cause of such high water bills. 

Apparently this had been happening all day long when they were not at home. Knowing that few would believe him, he taped a segment of the 'problem'  for posterity -- Click the link to see the video.  

 http://www.esnips.com/doc/5984dbee-9d87-4939-88e8-60a6a5619739