Something inconspicuous is there, but most people do not notice it. It does not catch the eye. A person can try to be inconspicuous by wearing plain clothes and walking quietly, so they melt into a crowd. A door, a mark, a small change can be inconspicuous too. The word is a bit formal. In everyday talk people more often say “hard to notice” or “easy to miss.” Though actually, it is not the same as invisible: the thing is not gone, and it is not hidden by magic. It is just not the kind of thing you look at twice.