They find one of Walden Macnair's arms. Just one. It was torn off in a way that left the shoulder partially attached and left hanging from what had been an Acromantula's web.
The students that discovered the grisly scene were horrified, unsure if the arms belonged to one of the defenders or a Death Eater. After a few days, Macnair's sister manages to id the arm because of the scar on Walden's left forefinger. The one she accidently gave him when they were playing around as children.
He never told anyone, but Hagrid was secretly pleased that Macnair had ended up a snack for one of the spiders. It felt like justice.