Monkey Sense is the second upgrade on the third path for the Wizard Monkey in BTD6. It allows the Wizard Monkey attack Camo Bloons. Unlike BTD5, this upgrade is along a different path to Fireball and all other fire-related upgrades. Visually, the monkey gains a pair of large thin black spectacles, along with a wide-brimmed wizard hat with purple crescents and a large purple gem on the front.
Simply speaking, this upgrade gives the Wizard Monkey with camo detection. In practice, Monkey Sense is useful with Path 2 for cheap Camo Lead popping power, or with Path 1 to pass walls and home energy at bloons.