tl;dr The fix is to increase the surface area of the contacts on the Shelly Flood sensor. See photo of fix, below.

Simple test: Put sensor contacts on a metal ruler (or other sheet metal) and it should definitely alarm.

Simple test 2: Put 3 coins (eg. US quarters = 25 cents) on a plate: One coin underneath each of the 3 metal contact points on the sensor. Place sensor on the 3 coins. Pour water on the plate. When water is touching all 3 coins (or perhaps 2 coins), sensor should alarm.

My tap water is pretty soft. It measures about 20 with a TDS (Total Dissolved Solids) meter.  As such, the Shelly Flood sensor does NOT trigger/alarm when it gets wet.  This means if there was a water leak, like a busted hose on the washing machine, the sensor would not fire and I would not get a notification. Very bad.

After some conversation on the Shelly support group on Facebook, I learned: If there aren’t enough minerals in the water, then there won’t be enough conductivity for the sensor to trigger. This is a known issue with the Shelly Flood.

Workaround is to increase the surface / contact area of the sensor’s contacts. Shelly said they are working on an add-on part that would solve this problem. But why wait? And their future mod may not be so great.

My fix: After soldering on some coiled bare wire (each about 7cm long), sensor alarms immediately with just 10ml (1 tablespoon) of water.