I don't know if it's a mod thing (I do have http://skyrim.nexusmods.com/downloads/file.php?id=6176) but ever since I taken the skeleton key from Mercer's dead body and haven't return it to Nocturnal I noticed that every so often I bump into a Nightingale Sentinel and they become rather hostile towards me. They're rather hard to kill since they do something to me right after they shoot me with one of their ice picks.

Is this common occurrence or is it a mod thing? To be more clear I can be anywhere. The latest location that I was at was near whiterun next to the demolished castle where I help fought the first dragon prior to the civil war.

They're hostile in my game too, I'm fairly certain it has to do with the mod as I haven't even touched the thieves guild aside from joining it.

I'm not using Wars in Skyrim, and I've never seen a Nightingale Sentinel anywhere outside of the Twilight Sepulchre quest. So it's a good bet the mod's doing it.