IMO, the sheaf was left behind accidently. When did the killer realize it was still at the CS? Hard to say, but most likely pretty early on he was aware he may have left DNA behind. That sheaf, as 10ofRods suggests, was probably analyzed immediately, yielding a male profile.
The killer may have thought (hoped) he left no DNA behind, but if so, he had to prevent his DNA from being obtained. He also had to attempt to eliminate/remove any possible victim DNA from his person, car, apartment.
This would involve making sure no water bottles, Starbucks cups, napkins, food etc were ever thrown away in public trash cans (office, cafeteria, classroom). Trash would have to be hoarded, if you will. But how to privately dispose? Place is small bags, and randomly leave around town at night when few to no witnesses. This strategy would also be used for disposing of trash from car and apartment cleaning. I believe, IMOO, this behavior prevented LE from obtaining his DNA sooner, as he was identified as a POI as early as late November. LE could not yet search his car, office, apartment. Where was he putting his trash?