In mission 15, where you have to kill the 11 guards without being noticed, even if I just stand still when getting out of the elevator, after a while the alarm goes off.
If I walk towards the stairs, there is a woman talking with a black woman who instantly goes in alerted mode as she sees me.
As an experiment, if I just stand still behind that woman, the guard pointed in my direction, after a while, goes into alerted mode instead.
If I become invisible, enter the first locked room, the guard hears noise and follows me, I blacken the windows and I disable him, the alarm goes off.
If I talk with the second guard you meet, guess what, when the talk ends the alarm goes off.

Early on I went to the upper floor with the elevator and let the searching timer go to 100% as a robot was still looking for me, came back down and the issue remained.
I tried remaining invisible for a while, the alarm goes off.

I tried killing the two women talking in front of the stairs while invisible, of course the alarm goes off.

I checked on other people playing the mission, and it goes smoothly without issues, I walk 10 meters and the alarm goes off.