Officer Ayala is a law enforcement officer. A trained professional. He does this kind of stuff, you know, for a living. Absent any really convincing evidence, I'm going to take a trained LEO's word about what he saw.
I don't think the question is 'were Martin's hands found beneath him'? I think the question is, 'how did Martin's hands get beneath him, since we have witnesses putting Martins' hands out to the side, and a LEO saying that Martins' hands were beneath him.