Some time ago our water softener had a defect and the morning of the cellar (about 64 square meters) was 10 centimeters deep underwater.
Shortly afterwards came out that our developers did not work reliably when installing the shower tray. The tub was not properly secured to the wall and over time the silicone has become leaky. Result of the next water damage.
After according to our neighbors ultimately the return flap for the sink in the basement did not work in a heavy rain as were spared luckily we now have three of these water detector:
One under the sink in the basement, next to a water softener and the third under the shower tray behind the trapdoor.
Another is still in the kitchen under the sink and / or dishwasher safe than sorry.
At first I had only just ordered a detector. After I tested this and the result has convinced me, I have more ordered. With the unit price of 9.99 euros is excellent value for money and a very sensible investment to detect water damage at an early stage and to avoid them completely. In our experience, I am annoyed that we have not already earlier before the damage the usefulness of the indicator - has been consciously.