I use a Homeseer Z-Wave Light Sensor. It's both battery operated (3 AAA, I believe) or Powered (USB). The batteries last me about a year. The device is extremely reliable, I've never had a problem with them in over 5 years. The same device has a swappable water leak sensor too - you tell hubitat what sensor you have in the preferences.
I use exactly for the scenario you describe - you just need to stick the sensor to something with a small light or indicator that is always on. I use my Rachio Irrigation system.
I also use another one to monitor my Washing machine and dryer to tell me when they are done.
Both scenarios are quick, easy and very reliable.
