Because you can use chatgpt without an account.

It is very easy to filter those out from the output of GPT, though, using basic UNIX utilities. In fact, many methods don't survive reformatting or copy-pasting, not requiring filtering at all.

It is a very basic watermark technique (text steganography) if it indeed is supposed to be one.

A more advanced one would be a linguistic (grammar-based) one, but I am not going to give any more ideas. :D

Probably won't show up in source code at all, though nothing of value would be lost if they did!

    $2.5
    billion

    $2.5 billion

But should probably be removed when copying, especially code

They can when they can. Sometimes they just won't like a kid or be tired or just wrong and falsely accuse them. There's been plenty already. I had a kid in my class during some years that would brag to other kids that his mom wrote all his longer, non-math homeworks. You're only putting yourself back by cheating, when time comes for proper exams for university placement you won't have it available and you won't take anyone else's place. Most places nowadays won't even fail kids from passing to the next grade, so it doesn't even matter.