Can you name those others who had a legitimate right to be in 5A? Then your question can be answered.
I gave examples above eg cleaner(s)?, GNR officers, former holiday makers and I believe others also entered 5A on the eve of 3rd May eg translator Silvia Batista.
The forensic report states
The remaining 28 samples analysed, of which 17 were recovered from the apartment, showed mitochondrial DNA from different from each other, and distinct from those above.On the assumption 17 abductors didn't enter 5A then clearly some of the samples must have originated from those who had a 'legitimate right' to be in 5A.
The forensic report does not use the term 'legitimate right'. Its a term used by S&S as per Anthro's quoted excerpt.
I think 17 unidentified samples in a holiday let would be expected based on the high turnover. Also hairs collected from floors could have been transferred via any mops/brushes used by the cleaner and/or transferred under foot from the soles of footwear.