If the dog was used to lots of random people being around at parties, he was probably desensitized and may not have made much of a fuss if someone came crawling out of the attic. Even if he did, DM said she thought she heard someone playing with the dog upstairs, and that could have been part of what was actually happening. He could have snuck in while Murphy was not there. I think I remember reading that Kaylee and Maddie were seen on video taking him for a walk after they got home from the Grub Truck. I could be misremembering though.That wouldn't work if the dog was in there, because if the perpetrator had been in that room, it would be considered part of the crime scene, and we know Murphy was not in the crime scene. If he was somewhere else in the home, fair enough.
MOO
The police did not find any other DNA in the house besides the sheath that we know of, so who's to say he wasn't in that room and did not leave anything behind like in the rest of the house? Maybe they didn't even test that area.
All opinion and speculation on my part.