Hi, I'm playing the first mission of the DLC and I notice that the three enemies that stand guard outside the torture room reappeared alive after I knocked them out and hid their bodies. It wouldn't be a problem if another enemy (I don't know which one) reappeared dead and the other ones are alert, it's odd because I don't kill anybody. So I think I won't get the ghost achievement.

Not to be obvious but once you've spoken to rothwild and deal with him however you see fit a few new guards enter the building.
I know this mite not be your issue just it's 3 guards appear and they hang around the main door entry/ interrogation area.
If this isn't the issue then sorry I have no idea