I took a class with a professor and really liked his teaching. Since then, we've had several conversations and I've gone to him for advice once. While I don't work with him and I don't know if he'd necessarily consider me a friend, I do feel like we've had a number of positive interactions.
He's going on sabbatical within the next few months, and I'm (crossed fingers) on track to graduate this next May, which means that there's a fairly high probability that I will be gone before he returns.
It feels premature to be saying goodbye to him now, around a year before I'm (possibly) leaving. If I graduate on time and he isn't back, do I just shoot him an e-mail which seems a little impersonal? Leave a handwritten note in his mailbox? Or is that really weird?
Is there a standard way of saying goodbye when I know there's a decent chance that I won't get to do it in-person a year from now? Or is it just normal that professors don't expect/aren't surprised when students they know are gone when they come back?