An enemy has been alerted once all three bars have been filled up over their heads, and the bars break. If an enemy has all three bars filled up, then the enemy has not yet been alerted to your presence; they just know something is wrong. This trophy is nowhere near as bad as it sounds, and should be combined with Redemptive Path. You will know at the end of each mission if you never alerted someone as a box will be checked that says "ghost, never detected". If you need help with this trophy, please see my pure stealth walkthrough for a text walkthrough as well as videos.