In both Payne's statement (arrest warrant/PCA) and Blaker's (search warrant), it states that "Upon our arrival, the Idaho State Police (lSP) Forensic Team was on scene and was ' preparing to begin processing the scene." Payne and Blaker stated that they were there to assist with the processing of the scene. Neither man states that they saw the knife sheath upon their initial basic observation of the room. Both indicated that knowledge of the presence of the knife sheath was acquired later: Payne through observation, Blaker through being told by ISP investigators.
The bodies would not have been moved before Payne and Blaker arrived there. ISP forensics had not yet begun their work. They would want Payne or Blaker there before they did any direct manipulation of the bodies and the immediate area where their bodies were. After Payne and Blaker got the cursory walkthrough of both crime scene rooms, then ISP forensics would begin its work in conjunction with them--hence the use of the term "later" as in after the initial walkthrough. ISP forensics team would be the one to move the sheets significantly and/or begin to shift the bodies of Maddie and Kaylee with Payne observing. Blaker's statement indicates he was not present in the room when that occurred by saying he was told later by ISP--maybe he's in Xana's room with some of the ISP forensics while this is going on or was elsewhere on the premises directing set up of security (another one of their stated reasons for being there). If Payne is standing there watching ISP move the covers and/or bodies, at that point he is in place to directly observe the sheath becoming visible and register that as information.