You responded to your own post? Nate, go home and say you were there. Make sure you get something done first, and try to break/end on something that you can ask a question about Wednesday, when everyone is back. That way, you can conveniently prove to your boss that you showed up, without directly addressing the attendence issue at hand.
Give them some line like "Yeah, I came in Monday, and I was going to go home when I saw that no one was here, but I figured I already made the trip so why not try to get something done. I got as far as I could on _______, then threw in the towel and called it a day."
If you tell them you were there for longer than you were, you create some options for yourself. I don't know how you're paid, but it can have a number of outcomes. If it's a holiday, you're probably not paid, but the hours can be 'comp' time - take off early Friday and mark the hours down like you're there. Maybe you get paid hourly, and these hours will show up in your paycheck. It all depends, but if no one's watching over you right now, you've got a lot of choices. It's the holidays - people are flexible.