I am at Lady Boyle's Last Party, I have an invitation and everything. After taking great pains to blend in with the guests, and quietly incapacitating most of the waitstaff and the watchman guarding the service stairs (by the rum fountain), I snuck upstairs to loot the joint. As far as I know, I was never spotted while I was up there, and I stashed all the watchmen's bodies in the attic where they couldn't be found.
However, when I come back downstairs, people start freaking out at the sight of me. I figured one of them must have seen me upstairs somehow, but after reloading my save a few times, I'm not so sure. I've re-loaded the same save several times, and sometimes everyone gets scared as soon as I come out of the service stair doorway, sometimes it doesn't happen until a few minutes later. It seems to be one of the Boyle sisters who notices me first each time but I can't say for sure. However, I've only talked to each sister once.
So, any idea what's going on? I've already finished the mission by slowing time, choking the appropriate sister, then Blinking her down to the basement, that allowed me to get away without killing anyone but I was still Detected even though I wouldn't have been otherwise. Just curious.