Hi, I've had a chance to work in the connected water field for a couple years, although not directly in leak detection. There are several companies that have products that may be interesting for what you are looking for. I have not actually used or tested any myself but have read through online reviews and articles. These are some I've seen.
Flo by Moen - Moen led a $28M investment in Flo in January. Flo has leak detection and a shutoff valve in a unit that you place on the inline pipe coming into the house.
LeakSmart - owned by Waxman. LeakSmart has leak detectors that you put on the floor around potential leak spots such as the washing machine or boiler. They connect to a Wifi hub, and LeakSmart also has a shutoff valve that you connect to the inline pipe and also integrates with the Wifi hub.
Phyn - owned by Belkin and Uponor. Similar to Flow, has a single system that you install on the inline pipe for leak detection and automatic shut off.
Streamlabs - has 2 products; one is an inline leak detector and the other is an inline leak detector + shutoff valve
Flume - this is a really unique product. You strap Flume on your existing water meter and it then monitors the water usage to detect leaks. The device connects to a Wifi hub. There is no shutoff option.
All of the leak detection + shutoff valve systems will cost $500+ and may require a subscription. They'll also require a plumber for installation. Flume is around $200 and is DYI, although it doesn't have a shutoff option.
One question for you all - all of these systems require a connection to Wifi. How would you connect the leak detector to Wifi in a rental? Would you setup your own Wifi network or would you connect it to a tenant's Wifi?
Thanks,
Len