Move Silently (Dex)
The Move Silently skill is used once the Sneak feat is activated. A sneaking character can be detected with a successful Listen check. Enemies that hear you may show a red question mark above their head, and then walk over to the noise's source. To escape detection, you'll need to be more quiet and move away from the enemy. It is subject to Armor Check Penalty.
In contrast, an invisible creature is now effectively immune from Spot checks if greater than 2 meters (a human body length). However, invisibility does not grant silent movement. While running or walking (not in Sneak) an invisible player has a Move Silently check of zero and can be easily detected.