Sneak check
From Arkham Horror Wiki
The Sneak check is a type of Skill check typically presented by encounter cards. It represents a test of the Investigator's stealthiness in the situation presented in the encounter.
A Sneak check uses the value of the Investigator's Sneak skill, plus any adjustments that the Investigator would receive from skill cards, allies, and Mythos card effects, plus any adjustment specified by this specific check. Unless otherwise specified, a single success is enough to pass the check.
Note: A Sneak check is not the same as an Evade check and therefore cannot be improved with the use of any bonuses specific to evading monsters.