Undetected means not noticed, discovered, or found. When something goes undetected, it escapes attention completely, like a spelling mistake that slips past three careful readers or a small crack in a foundation that builders fail to spot during inspection.

To detect something means to discover or notice it, often something hidden or hard to see. So undetected means the opposite: remaining hidden, unseen, or unfound.

A student might let an undetected error stay in their essay, not because they're careless, but because they genuinely didn't see it. A submarine tries to pass through enemy waters undetected, using stealth to avoid being spotted by radar or sonar. Scientists worry when diseases spread undetected before symptoms appear, because people don't know they need treatment.

Going undetected isn't always about sneaking around. Sometimes important things simply escape notice because they're subtle or because we're not looking in the right place. A gas leak might go undetected until someone notices the smell. A talented musician might perform undetected in subway stations for years before someone recognizes their ability. The key idea is that something exists or happens, but nobody notices or discovers it.