That door is opened by getting the quest "Louis' Stash" from Merril. When you confront the drunks in the bar..you have to make like its no big deal when Lieutenant Louis comes in and breaks it up.
You can then go to 2nd floor of the guardhouse and get the quest from Merril. The belt you find in the last room to finish the quest has +1 to Evade on it. It all hinges on NOT getting the drunks in trouble with the Lieutenant untill AFTER you get the quest DONE from Merril!
