This is a homicide by beating according to the Doe Network page, with blunt force trauma to the head having been the official COD, and yet the body also had signs of lacerations, even mutilation (acc. to NamUs), to the face, scalp and hands. This means that the killer probably both beat and used a knife during their attack on the decedent. The knife / weapon may have been disposed of since then. But this indicates that wherever the murder took place, there was likely some blood left behind. You can clean blood from many indoor surfaces, but oftentimes, the underlying particles are still present, particularly if it's soaked into some kind of textile or porous surface, i.e. concrete or tile.