Had a leak under the sink in our kitchen. I don’t want to imagine what this would have meant on a larger scale, compared the 20€ I spent on the sensor and the 10mins it costed me to install it.
Looks like an Aqara leak sensor. Zigbee - you generally won't find Z-wave devices for this cheap. They were around US$15 on Amazon when I last bought some in 2023, and a dollar or two cheaper on Aliexpress.
Pretty basic construction. There's two exposed terminals at the bottom, and when they connect electrically, it alerts. If you're clever, they can be used for a bunch of things, not just leak sensing (eg https://www.homeautomationguy.io/blog/making-my-own-bed-sensor).
A few years ago the hose on our washing machine split and we didn’t realise until water started coming out from under the units. Thankfully damage was minimal but it was a big pain to dry out.
I’ve had some of those Aqara leak sensors in place since as a precaution.
Just remember to change the batteries regularly! Easy to forget them when they are out of sight.
It sucks when you have a leak sensor, estimate the likeliest place that water will appear, miss it by a few inches thanks to the way water goes where it wants, and you end up having to manually catch the leak.