Marcus didn't give the decision to come to the dance too much thought. He never did think about any decisions, really. When he thought he should do something, he did it - and if it ended up as a wrong decision, oh well. It could have ended the better way as well.
So now he was entering the Great Hall, dressed up in black pants and a white shirt - another thing he didn't give much thought to. It didn't bother him that most of the people around were dressed in fancy robes, he never cared about it. He wore what he felt comfortable in, and only rarely thought about how appropriate (or inappropriate) it was.
He quickly scanned the place - the four tables were gone, of course, but that didn't mean he was going to forget where he belonged. He graduated already, but he still was a Slytherin.
Then his eyes fell on someone he... well, at least wasn't expecting to see so early again. Seemed that Marcus wasn't the only one who wasn't student or teacher and showed up here anyway.