You blend into your surroundings.

At-Will   ✦     Martial
Immediate Interrupt      Personal

Prerequisite: You must have training in Stealth.

Trigger: You are hidden and lose cover or concealment against an enemy.

Effect: You make a Stealth check. If your check beats the triggering enemy's passive Perception, you remain hidden from it, and until the end of your next turn you can remain hidden from it without needing any cover or concealment.

Update (10/3/2011)
Updated in Class Compendium.

Published in Player's Handbook, page(s) 120, Class Compendium.