You haven't included the possibility that the item was deliberately planted, having been already smeared with Raff's DNA. Given that the investigators on the YouTube video act as though they have prior knowledge of its significance, before any testing is made, then I don't think that can be considered particularly unlikely, either.
Antony, thank you for adding the above paragraph to your larger recent post. Thank you because it gives me the opportunity to add an observation that I have posted before, a month or so ago. My observation concerns Stefanoni's unique trip back to the crime scene to collect the bra clasp and what was going on with the police in mid-Descember when Stefanoni stepped in to rescue a bad case.
The Italian police began monitoring the phone conversatons of Sollecito family members from early November 2007. In all, they monitored 39,000 phone conversatons. The police and prosecution knew through intercepts in late November and early December 2007 that Raffaele's famiy was searching stores in different towns in Italy to find an example of the shoe model that Rudy wore. In early December the family found and acquired the shoe model. The sole of Rudy's shoe is noticeably different from Raffaele's shoe sole, proving that the police claim that bloody shoeprints at the crime scene were made by Raffaele's shoe is false. The police were about to be exposed and embarrased.
From early December when Raffaele's family found Rudy's shoe model, through mid-December and later, Raffaele's father, uncle, and sister repeatedly talked by phone about how and when to publicly reveal that police claims that Raffaele was at the crime scene are false.
On December 18 2007 police scientist Dr. Patricia Stefanoni, based in Rome, returned with her entourage to the crime scene in Perugia to collect the bra clasp. This was the specific purpose of the trip (defense were notified in advance). Stafanoni's entourage included lab assistants and camera and lighting technicians. The Perugia police were of course involved as well, since they control access to the "sealed" crime scene and provided support to Stefanoni and her crew. By arrangement, defense representatives were outside in a police van observing Stefanoni by live video. This was a triumphant event for Stefanoni to recover something sensational and key; she did not send an assitant instead. Don't know if police managers or Prosecutor Mignini feted/dined with her during her return to Perugia.
I contend that there is no way Dr. Stefanoni returned to Perugia with her entourage of assistants, supported by the Perugia police to reenter the "sealed" crime scene, set up lighting and video cameras inside the house, dealt with defense observers in the police monitor-equipped van, went into the house, located the missing bra clasp, triumphenly held it up for the camera, placed it down on the floor to be photographed in a different location from where it was actually recovered - no way did Stefanoni do all this on the off chance - the tiny, itsy bitsy chance that the bra clasp might, just might, have Raffaele's DNA on it. No way in hell was Stefanoni going to do all that on camera and discover in her lab that it was clean. I contend that Stefanoni planted Raffaele's DNA on the bra clasp.
What Stefanoni did not realize beforehand was that as she tested it her machine data output revealed that the bra clasp had DNA evidence of several other males on it. Those data records of other males are supressed (withheld) by Stefanoni. And to prevent the bra clasp from being examined a second time, Stefanoni "stored" the cloth-and-metal bra clasp by immersing it in liquid in a storage container, thus destroying the fabric and metal clasp.
