When in the bathroom I'll flush the toilet right there.  

To flush out private or side agreements or other details. 

We have three relatives visit us every year and I can see a substantial increase in our water and sewage bill even thought they only stay with use for a week or two due to the extra baths, cooking and toilet use.Washing cars or neighbors stealing water from a faucet can also be a problem in certain areas.If you can find a good time to check then check the meter on the street, make sure there is no water running in the unit and that no one will turn on or flush anything at all, let it sit 30 minutes or so and see if the meter goes up which would indicate a leak somewhere.The tenant may also be able to call the water company and plead their case (Not blaming you) that there was a leak that was out of their control and that they didn't realize it soon enough and see if they will give them a break on the bill, it depends on the water company but I have seen the water companies in my area give a break on the bill for certain circumstances.

I've done the yellow letter campaign thing for the last year and am looking for other effective means of flushing out sellers.

You can and will blow through $5k in Google AdWords or a Direct Mail Campaign and get absolutely nothing to show for it ::toilet flush::. 

If they are big guys, dump a lot, or use a lot of toilet paper, then make sure you have a toilet that flushes well and can handle the load.Show everyone who lives in the townhome (and regular visitors) the water turnoff that is behind to toilet and the water turnoffs that are under the sinks, in the laundry, etc.

Kept getting "ghost flushes". 

To the ideas expressed here, all it takes is for a random sample of FHA loans to find some of these and then have inspectors demand a much large investigation with that servicer and every loan at that servicer could be flushed out.  

SOB  I drive over to the shop and grab the torpedo heater , fill it with $ 50.00 of Kerosene , grab some buckets ( to flush toilets if need be)  I grab scrap wood and  tarps.  

Making them flush will be problematic due to construction tolerances.