Whatever the article was - spray of flowers, soft toy, card or note, then his hand/s would have touched it, and deposited perspiration, as it was a warm temperature.
It is my understanding from all the print media and news film that an interior door was removed from the Harriet St house - photos of a door wrapped in brown paper being carried from the house and placed in a police or forensic were in the media. Within a day or so, police announced that they had a DNA profile from inside the house of an unknown male. During the investigations, many of the male friends volunteered their DNA pursuant to a police request for volunteers. Accused was apparently not one of those.
A day or so after the door removal, police removed some of the memorial items from the memorial at the back fence of the property. It could be assumed that they took a few items, but likely already knew the closer male friends of the family who had NOT so far volunteered their DNA, and would have included any of those friends in the removals. (Could not be seen to take just one). It is also highly likely that they had other information which could have assisted in homing in on the accused from other friends of CR who may have been aware of the alleged infatuation with her. The DNA match from the memorial item to the unknown male DNA from inside the house (the door) would then have been the confirmation for the police - hence the charges. Also the blood spots outside the house, if they belonged to the accused would have been unknown DNA at the time they were profiled. But they may also have been blood from the victims which was tramped into the street, and would not have been unknown DNA. I don't think we know whose blood it actually was as I don't believe that information has been released.
Even if the memorial item which matched was contaminated from other items, it would likely have undergone the process of elimination - only needed to find a match for the unknown DNA profile from inside the house. But if it was contaminated, then that is an arguable point for the defence at trial.