My bedroom CO sensor and under fridge propane sensor say to test weekly. They must be hardwired in. I assume that the weekly test does the job because they scream and return in a few seconds to a solid green light. A red, non-green or alternating color light means the respective sensor is giving me the appropriate CO or propane warning, or the sensor should be replaced. That being said, we have two backup battery-powered sensors, a CO sensor and propane sensor. These have testing procedures too, as well as low battery warnings.
Your luck is the residue of your management.