The achievement unlocks when you reveal the 25th hidden object. In order to detect hidden chests, logs, wells, and other hidden treasures; you need a detect hidden skill of 2. You begin the game with a detect hidden skill of 0 so you will need two points. Note that if you choose to begin the game as a member of the Varani race, you start with a detect hidden skill of 1 and will need only one additional point.
You can increase your detect hidden skill by employing the services of a trainer--Arnick Arduna
at the Inn in Gorhart and Ambassador Odvar
in Ysa's Embassy Hall are basic trainers who will each increase your detect hidden skill by a point for a fee of 15,120 gold. You can also increase your detect hidden skill when you level up. Each time you do so you acquire a skill point. You can invest it in any one of nine skills--detect hidden being one of them.
The hidden treasures are depicted as chest icons on your mini- and local maps even if the treasure is not a chest but a pile, log, well, or the like. The detected object does not count until it is revealed. You reveal it as you approach the hidden object and press
when prompted. When you first approach a hidden object, your controller will vibrate. If you do not reveal the object at that time, your controller will not vibrate the next time you approach it but the hidden object will still be available to be looted.
Whenever you reveal the hidden object--whether immediately or later--your tally of detected hidden objects towards the achievement will increase. There are hundreds of hidden objects in the game. They usually contain above average loot so they are worth going after. It takes only a short time to reveal 25 of them. Nonetheless, you can use the achievement tracker if you feel the need to check your progress towards the achievement.