I'm a plumbing contractor. So I do install many different types of water monitor of cut-off devices.

1. In the crawl space areas or my buildings I have simple water detecting alarms that have an audio loud alarm. Tenants call me if they hear something.

2. I also have hooked up to my water heaters cut off devices so when they start to leak, it cuts off the supply to those water heaters.

Other than tenants abuse the water (My class B tenants uses 1/3 of my C tenants), the very common problem is toilet run.. Whenever I got a high bill, I just go schedule a walk through to check all the toilets.. If it is an older model, replace with the cheapest from homedepot($60).. That eliminated most of that.

I would love to use what Robert is doing.. Just wonder if the cost will justify that..

Home Depot has a $12 water alarm with a detachable cable and sensor.

https://www.homedepot.com/p/Basement-Watchdog-Battery-Operated-Water-Alarm-BWD-HWA/100038838

I like this one because the unit brain and battery can be elevated and only the sensor is placed on the surface, dirt, etc.

I use many different cut off devices for Water Heaters, Washing Machines, Dishwashers and Ice Makers. For everything except water heaters I use things like a "Flood Stop". But for water heaters I use a combination device that not only cuts off the water but also cuts off the gas to the water heater. This way if the pilot or burner gets blown out, there won't be a gas explosion. These devices range from $90 to $400.

For toilets, they make a new type of Shut-Off Vale with a connect supply flex for toilets. So when there is no back pressure -- the ballcock failed and water is just running -- then the water to the toilet gets turned off. All you have to do to reset it is to shut off the toilet supply and turn it back on....

OR you can get a Water Excess Valve and set it for 2 or 3 gallons for each toilet flush. If the water keeps on going above that, it will cut off your toilet. These valves are around $120 each.

Note that if you find a leak and fix it and report it to your local water company, they often will waive/reduce that fee as a one-time thing. This is fairly common, so likely worth the call to find out!

- John...

I also use the $12 alarms in basements and place them near water heaters.

I agree the biggest culprits are toilets. I'd like to know more about the toilet shut off valve that turns off on a running toilet. I went through 17 units this weekend and found 1 slow leaking flapper on a toilet. I expected to find more based on the water bill but I didn't.