Lot to cover, normally I'd do bulletpoints but sick and tired xD
Sebas *has* done something to go against them. They were there to gather info and not draw attention. Sebas did a GOOD thing and the RIGHT thing, but Nazarick and most of it's denizens don't care (and just don't understand) that cause they're all very much evil by design.
What Sebas did has drawn attention to them, and bad attention at that, the exact kind they didn't want. On top of that, Sebas hasn't even REPORTED this to Ainz and is keeping it secret. Actively withholding info about the situation because I think, deep down, he believes Ainz wouldn't understand. As you say though we'll see what happens there IN THE NEXT EPISODE yeeeeah (which has already been posted xD)