A play on the word 'espionage', only with lesbians involved.
Usually used in such a way where a woman (who is a lesbian) is inquiring as to a females sexual preference in a round about way (i.e a friend or indirect questioning).
Guy 1: Hey! You know that Savvas girl?
Guy 2: the Greek one?
Guy 1: Yeah! I heard her talking to my girlfriend...asking questions about her favorite cars...sounded like a case of lesbionage....
Guy 2: Aw, gay.