If something goes unnoticed, nobody sees it, hears it, or realises it is there. The most useful thing to learn about this word is the company it keeps: it almost always follows go, pass, or remain — go unnoticed, pass unnoticed, remain unnoticed. That pattern is unusual in English, because those verbs normally do not take an adjective like this, and it is why the word can feel odd in a sentence students build themselves. It can be about physical things — a small mistake in a long report can easily go unnoticed — or about people, and with people it often carries a quiet sadness: an old man who lives alone and whose absence goes unnoticed for a week. But it can also be exactly what someone wants: a thief hopes to slip in unnoticed, and a shy student is glad to get to the back row unnoticed. The word itself does not say whether that is good or bad; the situation says it. It also works before a noun, though less often — an unnoticed detail, an unnoticed error.