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Detecting hidden is used to reveal hidden traps and players. It has two modes: area detect and targeted detecting. When targeting the ground, all hidden players in that area can be revealed (depending on your detect hidden skill and their hiding). Floor traps are also uncovered, anyone can detect the traps in the dungeons but to find faction traps you've to be in a faction. The delay for area-detect is 1 second, and the range depends on your skill.

To detect traps on containers you've target the trap (This changed recently, trapped containers could be found by simply targeting the ground). Target-based detect has a delay of 11 seconds.

50.0 Detect hidden is required before you can use the remove trap skill.

Skill training

Passive gain: Detecting hidden can go up when picking a lock or removing a trap.
Active gain: Below 50 it's fastest to target the area where a hidden player is standing. In powerhour you can get to about 50 detect just by attempting to reveal someone with GM hiding. This doesn't work very well after 50 because of the anti macroing code: You must detect different targets to gain. To use this to GM you'd need hundreds of hidden players!
Active gain: Faction traps. You can join factions and then detect all the traps around towns and strongholds. You've the same problem here as with the last method, there is only a limited amount of traps to detect.
Active gain: Trapped boxes. Create a few hundred trapped boxes and then record a macro detecting them all. With 200-300 trapped boxes this shouldn't be much harder to GM than, for example, arms lore. It's going to be much slower though because of the 12 second delay.
UNVERIFIED: Make lots of trapped boxes and place them inside a chest. Place this chest on the ground and use detect hidden on it. Theoretically it should check all the boxes inside for traps, so you can check 100+ traps at once. I have not been able to get this to work.
UNVERIFIED: Passive gain through lockpicking: Since you can gain detect hidden by lockpicking, it may be possible to use the same trick that also works for anatomy and animal lore. You must have some lockpicking and an easy to open lockable box. Record locking/lockpicking the chest. Run this macro until you gain 0.1 detect, then lock your lockpicking skill. Now whenever you pick the chest, you should gain 0.1 detect hidden. This may also be affected by the target and/or location. Try it on a boat.

Tips & Tricks

Anyone can detect someone who just casted invisibility and has 0 hiding. Simply use the skill and target the hidden player to reveal him.
When using detect hidden in your house, or a house that you're friended to, all hidden creatures inside are automatically revealed.

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Version 1.0: Initial draft.