I had one student in a class on a Friday, then claim he had flu - and no, it was definitely much more serious than a cold - the following Monday (the day an assignment was due). He was back in my class on the Wednesday looking perfectly OK. I have had flu a few times and that recovery period was just not credible. He was completely busted when one of my colleagues told me he had seen scumweasel getting obnoxiously drunk in a bar near the university on the Saturday night, and one of his classmates let slip that he had taken part in a soccer tournament on the Sunday. We passed that case to the disciplinary panel.
What if the student didn't lie to *you*? Your case caught my attention because I just had one of my students lie about swine flu... to another professor. The story:
Before class started, I was chatting with the student about some of the points I was going to cover in class. I made a comment about how expensive custom papers are and hu said "Oh, I know! It's like, two hundred per paper!" After that, I just stared at him for a minute or so while I mulled over the meaning of his statement and he quickly followed up by saying "Oh, oh! No-it was a friend who got one. I'd never be able to afford one! Ha! It's actually a really funny story! This past Monday I got a call from him begging me to get up and go to school to vouch for his having swine flu. His teacher's a hardball about not accepting papers late and he the paper he bought was sent a day late so he needed the excuse. So I had to drive all the way down here on my day off just for him. Isn't that crazy?"
I didn't respond to hu's story because more students had trickled in and class was starting. By the time class had ended I'd forgotten all about the conversation.