a clean classmate

The class in our room before us had left a box of donut holes at the front of the room.  As my class was about to start, we passed around the box, enjoying the unexpected treat.
I finished mine as the professor started lecturing, and began opening a document on my laptop in which to take notes.
"Psst!"
The student behind me hailed me none-too-subtly.
I turned and glanced at him, trying not to disrupt class.
"Your hands are dirty!  You need this so you won't get germs on your laptop!"
Before I could respond he tossed me a small bottle of hand sanitizer.  Somewhat creeped out, I opened the bottle and upended it to put a bit in my palm.  Eager to be of service, my classmate mimed rubbing his hands together so that I would know what to do with the substance I now held.
Finished, I handed back the bottle and mouthed the words "Thank you," still a bit confused by his gesture.
Smiling, he held his hands up to his nose.
I frowned, confused.
He repeated the gesture, whispering, "smell your hands!"
I turned back to my laptop, somewhat disturbed.

He caught me after class.
"Did you recognize that smell?"  He asked eagerly.
"No, uh, no, I didn't."
"It's rosemary!"
Delighted with himself, he scurried away down the hall.