In a way, hiding in plain sight can be so easy that most of the time, we're not even doing it on purpose. All you need is for something either larger or more flamboyant nearby to steal the focus and you can suddenly get away with a lot.
However, your chances of deflecting discovery get a little worse when you're in the same shot as something eye-catching. Because the longer that photos exists, the more likely someone is to notice something subtly odd about it.
And that fact is the main reason why any of these photos' hidden secrets could ever see the light of day.