Tenant intentionally wasting water

[Ariel K.]

I have a 12-unit building and after going through a lot of leak checks, my PM and I have come to the conclusion that a tenant or tenants are wasting water - to the tune of 5,000 gallons a day (a normal day is around 800).  There is one meter, so we tried to look at putting a digital submeter, but every plumber says it won't work in our situation.  We know 3 of the units can't be the issue since they either moved in or out while this was happening.  We sent messages to all residents and 3 didn't answer, so it seems like that is the place to start, but does anyone have any ideas on how to figure this out given the limitations?