So I went back to re-watch the first episode again and noticed a few things now.
What do you see different about the drawer where Andrea gets the knife out of?
Here's a screen capture from when she's taking the knife out.
And here's a picture from Stone's investigator in Episode 5.
We know the window of time was small here so someone quickly walked up the stairs, grabbed a knife out of the open drawer then walk upstairs and kill Andrea. They also did not leave the knife behind. If the killer came in from the first level through the door with the broken gate (as we suspect), then they would have walked right past this drawer and taken a similar path that Nas took when he woke up and walked upstairs.