My empty tray results in a measurement

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.