One that I come to wonder about every little while is two students from my class at university.
The first, we never met in person. He was on all the attendance registers and paperwork for the class, but never showed up. The lecturers expressed as much surprise and confusion as the students, so he hadn't told them he'd fallen ill or anything. However, every month or so, he sent around a group email stating how sad he was to not have met us yet, and how he hoped to see us soon, so we all thought he'd finally be making an appearance. No such luck, and then each month gone brought another email. To make things even stranger, in spite of never attending lessons, completing assignments (that we know of - there were several group assignments involving presentations, he could have been submitting the essays) or taking the exams, we came into our second year (you had to pass the first year to be allowed to start the second) and his name was still on the register!
The second girl I actually met and got on quite well with. She and I had a fair amount in common and on occasion would work or study together. We had an assignment that required us to record a conversation between ourselves and another person (so we could transcribe it), and she offered to lend me her dictaphone, having already made a recording for her own transcript. I gratefully accepted, made my recording, saved it to my computer and came in the following Monday ready to return it... only for her to not be in our lesson. Okay, she's ill, I thought, except she wasn't in all week. I sent her a message on FB reminding her that I had her dictaphone, and got no response. She never came back to university. I attempted on several occasions to get her dictaphone back to her, but she stopped answering her phone and FB.
I still have the dictaphone...