I've got a fishing job from the thieves guild and it takes place in Markarth. The individual that I'm supposed to steal from is named Imedhnain, and he is located in Arnleif and Sons Trading Company. I have been unable to find him anywhere else but there, although he may leave at certain times of the day - I'm not sure. In any case, when I break in to Arnleif and Sons at night, as soon as I break in I am seen by him and Lisbet, as they're having a conversation at a counter directly in front of me. I've tried breaking in at different hours of the night, but every time they're there at the counter. Also, I've tried using an invisibility potion before lockpicking the door, but the potion seems to expire right when I enter. 

How exactly do I go about stealing from this fool if I get caught every time I break-in?