Fading

Fading is the art of hiding in plain sight. It's most commonly used when someone (usually a criminal) "fades into the crowd" to avoid detection. Fading into crowd involves concealing distinctive traits, clothing, or gear (or avoiding such things in the first place) and mirroring the crowd's demeanor and actions so that you become just another face in the crowd. The more advanced form of fading--typically only useful to spies, assassins, and thieves engaging in long jobs--is "fading into" a place. While being unremarkable is helpful hear as well, fading into a place relies at least as much on establishing oneself as a person (usually a serving person beneath the notice of the mark) who is supposed to be there. For public places, this can be achieved through patience and time served. Fading into a private locale often requires obtaining honest work there, or at the very least obtaining the proper attire and accouterments to look like you're engaged in honest work.
A related figure of speech involves saying that a person has "faded to" a particular location or obect (often a barstool). This simply means that the person in question is so ever-present that other regular visitors to the place tend to take about as much notice of the fader as they do of the furniture. This can sometime lead to the faded party overhearing valuable information.
When a person simply drifts away from an organization or social group without notice, they are said to have "faded out" of the group.