Actually, that "swamp" scene could be at the very end where Harry's chasing Snape across the lawn of Hogwarts. He does have the DA patrolling that night while he's with Dumbledore so that could explain the housecoat Ginny's wearing. And if you frame by frame it, I think it's Harry running through the grasses. It sounds Greybackish, but then he would be running there in that scene too. I love that "I"M FREAKING CRAZY!!!" face that Ralph Fiennes sports here. :D

Harry Potter and the Half-Blood Prince
Teaser Trailer
(1:42)