That was weird. I had a Samsung Water Leak sensor (one of the later ones with sensors also on top) get wet. I had a Basic Rule that triggered properly ...
When Water Leak Master Bath water leak detected ... Turn on * Alexa says Water Leak Master Bath
However, this same leak sensor is listed in HSM Water monitor, and Water was armed. But none of the actions for a water alert in HSM executed, leaving the main water valve open and no notifications. There were no events in the log related to HSM at the time. HSM failed me, fortunately with no bad consequences since I was home, and the "leak" was just the cleaning lady mopping the floor.
But wait, there's more. After this, I wiped off the moisture on the leak sensor, but the sensor didn't report dry (at least Hubitat didn't reflect that it had). I made really sure that the sensor was completely dry inside and out. I replaced the battery. And the sensor was correctly reporting temperature and battery events .. just not dry. So then I replaced the device type of Generic Zigbee Moisture Sensor with Virtual Moisture Sensor and saved. With this driver, I was able to force the dry state. Then I changed the driver back and it shows dry now (and I know that the sensor is sending events).