Undetected means nobody noticed. It almost always attaches to something that was there all along and slipped past whoever was supposed to catch it — a crack in a wall, a fault in a program, a disease in the body, someone sneaking through a border. The classic pattern is "go undetected" or "pass undetected", and that pattern does most of the work in real English: "the leak went undetected for months", "the error went undetected until the audit". You can also put it before a noun ("an undetected illness"), though that is rarer. Two shades to feel. First, it usually implies there was some system doing the looking — a scan, a guard, a test, a camera — so it is not just "nobody happened to see it"; it is "it beat the checking". Second, the thing undetected is nearly always bad or unwanted. English does have neutral uses in science ( "the signal remained undetected"), but in ordinary reading the word comes wrapped in a small sense of danger. It is a formal, written word: news, medicine, technology, security. In conversation people say "nobody noticed" or "it slipped through".