When Kimble places the murder weapon, which is wrapped in a towel, in the air duct he places it far enough into the duct that the towel is not visible from the floor. However when Lt. Sloan looks up at the vent cover for no reason the towel is right up against the vent so as to be clearly visible from the floor. However when Sloan opens the partially attached vent cover he has to reach well in to the air duct to retrieve the towel covered object.
Millie's boyfriend was seen by numerous people in the hallway, engaged in a seemingly hours-long, very public make-out session with Millie. But he then completely disappears from the story, and is not even mentioned by anyone as a possible suspect in the murder case.