Missing, not present when needed. The expression has been extended, especially by British middle-class speakers, from its original meaning in army jargon of 'absent without leave' to inexplicably absent, either with the implication of fleeing to avoid responsibilities, or wandering uncontrolled or running amok.

'Ollie's gone awol again; he disappeared with a bottle and no one's seen him for days.' (Recorded, upper-class youth, London, 1985).