Must've Left it Open
Lockpick 5 doors.

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Some doors are locked and you need to have at minimum, the Apprentice Locksmith skill. I highly recommend buying the Journeyman and Master Locksmith skills as soon as you can as opening doors without it can be pretty hard. So to open a locked door, you first need to have some lockpicks. Lockpicks can be found on dead enemies, but the easiest way to get them is by buying them from merchants. Most palaces and mansions have locked doors, and you will encounter lots on the main story missions and the co-op ones. Now, you have to do a little lock picking mini-game. The aim is to press when the line gets into the "safe" zone. Some locks require you to do this twice, sometimes even 3 times, but most of the time you just need to do it once. If you miss the "safe" zone, a lockpick will break. All you need to do for this trophy is open 5 doors.