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Dude, just getting passed while going down the front straight can be an "unexpected area" for a new track rider.
"Unexpected" doesn't necessarily mean "dangerous" or "illegal" in any way shape or form. And it doesn't necessarily have to SCARE them or startle them, either, it can just be "Oh... hmm... didn't expect to get passed there. Ok."
I dare say, if you talked to your beginner group and asked them to count how many times they got passed when and where they didn't really expect it, that number would probably be much bigger than you'd think. (With that said, I probably WOULDN'T ask them to do that as they've got enough to think about already, but if they did, I bet you'd be surprised with the results.)