Seems to be automatic. When people are mildly stunned, they'll often go "어" (eoh).
The teacher would ask, "What's the capital of X?", and she'd respond confidently (and correctly), "Okay! The capital of X is Y."
Not sure if it was conscious on her part or not. I'm sure it came from the spotlight, since she was otherwise "normal" (not that there necessarily is such a thing).
> Everywhere this word appears to be a simple syllable with a low-front central vowel, glottal onset consonant if any, and questioning intonation.
My wife's parents have a similar thing in English as an aside. `Huh` indicates that they've heard something novel, `huh?` indicates that they didn't hear you.