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.
Best Answer
If anyone sees you while you're trespassing (upstairs) everyone will turn hostile (one person seeing you = everyone, oddly enough), I'd bet that's what's happening. It can be tricky to come back downstairs without being spotted (even when using tricks like timestop and blink people seem to see me). Note that you can totally kill/knock out the upstairs guards without alerting everyone else if you're careful; every playthrough I've done I've KOed or killed the guards upstairs and I usually get out scott-free.
Try exiting the downstairs via a more discrete way: you can leave via the balcony (the door to the outside from the first bedroom after the stairs) and the guard outside won't "detect" you (he's also easy to knock out or kill, but it's pretty unnecessary). He just politely asks you to stop trespassing. Drop gracefully from the balcony and no one should turn hostile (note that falling on someone's head, as I did the first time, counts as an attack).
Also note the basement, oddly, doesn't count as trespassing, so I'd poke around all the "safe" areas before going upstairs, and making a real save before going upstairs, since it's usually when things go south in this mission.