Archaeologist

Walkthrough
Locations for the Enigma of Kirkwall secret messages:

ACT I 1. The Bone Pit (under a group of boulders in the starting entrance) 2. The Gallows 3. Lowtown Alienage (Only at night) 4. Viscount’s Keep (In one of the filing cabinets in the little office.)
 * The message at the Viscount's Keep in Act 1 is in the room at the NW corner at the top of the map that can be accessed after the events of The Unbidden Rescue. It is possible that the icon to select the Enigma message does not show up when using the radial menu. Try clicking the selection button while facing the vase between the two paintings on the north wall. (bug confirmed on PS3, if you cannot select it, stand directly in front of the vase and move the radial menu around, you may unlock the codex entry without actually clicking, it will appear on the screen.)

ACT 2 1. Chantry (top right side, on the table.) 2. Darktown 3. Docks of the Qunari Compound 4. Gallows Dungeon during Dissent quest
 * It is found in the corner just past the exit past the Poison shop
 * After plot events involving Anders, the Enigma of Kirkwall messsage can be found in a clearing west of the exit.

ACT 3 (all found during quests) 1. Justice (quest) (In the sewers). 2. Found in Templar Hill (During On The Loose: 2nd room on the right (the one after Meredith's room), look at the lamp on the wall next to the door. 3. ‘Last Straw’ on the docks. 4. ‘Gamlen’s Greatest Treasure’.
 * During the quest you will be asked to head to Smetty's Fish Guttery at the Docks (Night). Once you are inside the Fish Guttery, head to the left and down the ramp to find this in a pile of rubble between the two boats.

Result
You can check the Journal>Codex>Lore>The Enigma of Kirkwall to see new signature entries for the secret letters you have found.

You will know how many you have by the number of signatures in the entry.

Rewards
By finding at least 3 of these notes in each act, you will earn the 'Archeologist' achievement.