Back of the toilets.

Quite a number of leaks come from the braided pipe feeding water to your cistern, as they often only have a five-year lifetime

Mine are basically under every water-using appliance, plus some extra ones in the basement in places where there is a risk of water ingress.

I've also built a sensor (HC-SR04, ESP32, ESPHome, Home Assistant) to monitor the water level in my sump pit so that I can make sure the pumps are working.

Next to any condensate pumps, like you'd have for air conditioning, maybe for a dehumidifier.