Then again, Arcadia's may be glitched in other ways. I mean, the entire time I was robbing her, she was sound asleep in her bed, my stealth indicator claimed I was "hidden" and she's chatting me up as though she was standing behind her counter. "Oh, you don't look so good." "Hmm, that may be ataxia." "Let me know if you need anything."
I've had something similar happen, though with a different NPC. Can't remember for sure, but I think it was the spiced wine merchant in Solitude. I snuck into this someone's house, hidden the whole time. I'm 100% sure nobody spotted me. Despite that, I heard a faint voice shouting "you need to leave!". The stealth indicator kept showing me as being hidden, and the shouting continued, so I followed the voice. What I found was this woman laying in her bed, seemingly sleeping, while repeating "You need to leave!", "You aren't supposed to be here!". She didn't spot me however, being
asleep and all, so I just carried on liberating the house from everything valuable. I didn't get any thugs after me after that, so the quest engine apparently didn't think she spotted me either.