...hey. So some dog brought the murder weapon to us...

That means the murderer took it from the crime scene, because the dog couldn't have gotten it from the locked room. So that means they either meant to plant it somewhere... or keep it. It seems a lot more obvious that they'd want to put it somewhere it shouldn't be... right? So that's another reason they wouldn't want people in that room, to find out it didn't really come from... wherever they put it.