Today we (professors + teaching assistants) proctored a midterm exam for a class of about 80 students. There was undoubtedly a "hardest" question on the exam, since nearly the entire classroom of students asked us how to proceed with that question. To be fair, we didn't give any hints, but it was clear that one had to use a definition to be able to proceed.
When we collected the exams, all of the exam booklets had that question unanswered -- except for one student's exam. And this was the only student who asked to go to the bathroom. I gave him permission to go, but I did not ask for his phone, which I now highly regret.
What can we do in this situation, in which I strongly suspect that this student went to the bathroom to look up a definition on his phone, so that he could answer the question correctly?
We still have to look at every exam to be sure that only one student got it right (the bathroom student), but if this were indeed the case, do we have any power to accuse him of cheating, or, have we missed our chance by not asking for him to leave his phone in the room?
I feel a big injustice will have been done to the rest of the honest students, if we let this one slide ...