If they were close to the body, they would have been under the blanket.
CST Smith arrived at 8:03 PM and Malphurs arrived at 9:45 PM. Santiago arrived at 8:10 PM, shortly after Smith and well before Malphurs. He says (16/184):
On February 26, 2012, I was notified to respond to the area of Retreat View Circle reference a shooting. I then notified Inv. Singleton to also respond. Upon my arrival and entering the crime scene, I observed a black male, later identified as, Trayvon Martin lying on the ground with his face up with his head to the west Martin was wearing a grey sweatshirt and light colored shorts and white tennis shoes.
Smith photographed the scene, so quite likely had the yellow covering removed while she photographed areas around the body.
In any event, I think it somewhat unlikely that the headphones would end up loose on the ground so close to Martin that they'd be covered with the same blanket. The altercation apparently moved some distance, and Martin moved after he was shot. If the headphones were to fall to the ground, I think they'd probably have fallen earlier, so there'd be some luck involved in Martin ending up right next to them.
Malpurs was listed in the first discovery as a "B" witness. That seems like a strange designation for someone so much involved in the investigation, especially someone who didn't write a report. "B" witnesses normally can only be deposed by leave of the court. I don't know if the state changed her to an "A" witness, or if she was deposed by mutual consent between the defense and prosecution.
(In passing, I assume Tara Malphurs is the same person as Tara Clark, with perhaps a change in marital status.)