Thymolphthalein is an acid-base indicator commonly used as "disappearing ink." It transitions at around pH 10. Below this pH, it's invisible, but above this pH it's blue.
Sodium hydroxide (very basic) is added to thymolphthalein to keep the ink blue, but CO2 from the air neutralizes the ink and it becomes colorless. If something basic is added to the secret note, it becomes blue (and visible again!)