incognita

A woman who is unknown or in disguise.

The state of being in disguise.

Of a woman: without being known; in an assumed character, or under an assumed title; in disguise; feminine of incognito.

Of all places, London is the best for an incognita acquaintance; cards may be exchanged to all eternity without a meeting, and the various circles revolve like planets in their different systems, utterly unconscious of the means and modes of each other's existence.