We are used to the level in the measuring tube holding artificially high compared to the level in the tank, since the tube's level doesn't drop as oil is sucked from the tank - until we hold down the red button to equal them out. Without the red button, a breakage in the (relatively vulnerable) measuring tube could result in the entire tank of oil leaking out.
So what happens in cold weather? If you last checked your level in warm conditions and on a cold day you go to look, the measure will record a much lower level than is in the tank. When you hold down the red button, the oil level in the tube shoots up! When we checked ours, the level rose a good 15cm (6 inches).
Before you panic and think that someone's syphoned off a chunk of your (very expensive) oil, hold down the red button and make sure that the level is true.