<snipped for focus>
How did that word ‘always’ slip into the discussion? I believe that people have made the point, over and over, that each case is different.
In this case, however, single-person DNA, on the snap of the sheath, indicates that the person did actually touch the snap of the sheath. (Compare it to the situation where I shake someone’s hand vigorously, and a few minutes later I open a door. Logically, there might be DNA on the door handle that is a mixture of my DNA and the person with whom I shook hands.
Different circumstances, different evidence.
MOO