Etiquette/social norms for a grad student toward a faculty going on sabbatical 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?
When a professor goes on sabbatical for a year they know full well that students will graduate in the meantime. They have seen this happen from both sides (they were students once and their professors took sabbaticals as well.) There is no standard way to say good-bye. A note,  either electronic or physical, delivered before you leave is appropriate. So is asking the professor in question if they will be attending any of the graduation/end of year ceremonies that you will be having. Or you can simply say good-bye the last time you are going to see them before they leave since it may be the last time you see them. 

General advice for keeping it from being "really weird" is to keep whatever you do short. Rambling will make it get uncomfortable for both of you. You have three basic things you might want to say: thank you, good-bye, best wishes for your sabbatical. You don't need a lot of words to get through those ideas.