The ability to move about unseen is not foolproof. While they can’t be seen, invisible creatures can be heard, smelled, or felt. Invisibility makes a creature undetectable by vision, including darkvision. Invisibility does not, by itself, make a creature immune to critical hits, but it does make the creature immune to extra damage from being a ranger’s favored enemy and from sneak attacks. A creature can generally notice the presence of an active invisible creature within 30 feet with a DC 20 Spot check. A creature with the scent ability can detect an invisible creature as it would a visible one.