They look so normal that they don't stand out, so it's harder to tell them apart from other things.Like if you were hiding from someone on a crowd of people. Since there's so many people, you don't have to do anything weird in order to be hard to find.Another example is like if you were looking for a specific book in a library. The book you are looking for is hiding in plain sight, since they aren't hidden away in a secret spot. It's just hard to find a book in a place where there's so many of them.
Even the mystery which has been hid from ages and from generations, but now is made manifest to his saints: the mystery. Romans 16:25,26 Now to him that is of power to stablish you according to my gospel, and the preaching of Jesus Christ, according to the revelation of the mystery, which was kept secret since the world began. From Longman Dictionary of Contemporary English in plain sight in plain sight American English if something is in plain sight, it is easy to see or notice, especially when it should be hidden Don’t leave your valuables in plain sight. → plain Examples from the Corpus in plain sight. I couldn't believe they'd left the drugs and needles. Balance sheet example.