I suppose he might have handled the sheath carefully always wearing gloves
but...
Ooops!
The very first day (in the shop?) he excitedly opened it with bare hands.
Only once -
but his DNA happily nestled under the snap.
Provided, of course, that it was he who left it at the crime scene.
Well, this is my imagination again
JMO
But he had to touch gloves to put them on.
Gloves are great at keeping fingerprints off things, not so great at keeping all epithelial cells off things. We shed 500,000,000 a day or thereabouts. He should have worn a respirator, goggles, and surgical hat, shoes, etc and never touched his own clothing except for what was laundered - if he wanted to keep all DNA off his sheath.
You breathe out some epithelial cells, for example. Scientists are currently using breath to attempt to diagnose certain lung diseases quickly and easily - from the epithelial cells emitted in breathing.
A mask will capture some of these cell, so he would have had to mask up each time he looked at his knife - but even then, masks are not 100%. 1% of 500,000,000 daily cells is a lot (and the air around us has our epithelial cells floating around until they settle somewhere).
Most people unconsciously touch their own faces several times an hour - he'd have had to practice his best gloving technique and change gloves frequently. I bet he did not do that. And everyone breathes. I doubt he was wearing a mask while he drove in his car, but perhaps he did.
As the old saying goes, something always leaves with the murderer and something is always left behind.
I bet BK is sitting in his cell wishing he had in fact opened his knife packaging while wearing gloves - he could have diminished the amount. He'd have had to change gloves frequently. DNA is not destroyed by soap, bleach or alcohol, so he couldn't just wipe his gloved fingers with wipes - but it would have helped to do that too, by dispersing some of the cells onto the wipe.
I bet, instead, that he practiced with the knife with his bare hands. He thought he'd be tossing both knife and sheath.
IMO. Speculation, of course.