I tested an empty tray (no oil or plain oil) and tCheck showed a measurement result. Why?!
tCheck assumes that there is some liquid in the tray during measurement and measures the darkness of the liquid. Liquids in the tray act as a lens and direct more light onto the sensors than air. This is a little like looking through an empty jar versus a jar filled with water. The filled jar magnifies the image slightly because of the way light bends through the water.