Be persistent!

Maybe the owner has passed away and the heirs are just trying to keep on running the business, or maybe the property has too much deferred maintenance and the owner doesn't want to pay to update it or list it out of embarrassment.Get out there and get uncomfortable, you'll find something with enough persistence. 

The key, as with any kind of real estate, is to remain persistent and patient. 

The key is persistence, integrity, and transparency.

Just want to wish you luck and remind you PERSISTANCE IS KEY!

Then persistently pushing usually moves things along.But I'd certainly get them served and start the clock ticking for worst case scenario.Good luck!

Take things one step at a time, RE is not a get rich quick scheme for 99.9% of people, and is an asset class that rewards patient and persistent individuals with a desire to always keep learning.

Downward pressure from volume will persist.

Beyond this, expectations and persistence cure everything. 

In that case, you might try sending out a letter to all tenants stating that you have not located the problem and if the problem persists you will have to "address the unusual water expenses" if someone has moved a bunch of people in, then another tenant will probably rat them out so that their own rent doesn't increase-- if not then you could submeter and find out where the additional usage is.