"Also in the clip you can clearly hear discord voice-chat. There are some voices with clearly don't add up, Maybe im wrong"
No, in the clip, you can clearly see the exact opposite. Every voice that was heard lines up with a voice box popping up in the bottom right.
With that said, it does seem like they were peeking through walls whether or not they had "prior map knowledge."
Before the door was open, "he's on the right, he's on the right side watch out." So unless he made any noise beforehand.. seems like glitching through the walls.