Into thin air used to refer to the state of being invisible or non-existent.
the thin red line the British army (in reference to the traditional scarlet uniform); the phrase first occurs in the war correspondent William Howard Russell's book The British Expedition to the Crimea (1877) of the Russians charging the British at Balaclava. Russell's original dispatch to The Times, 14 November 1854, had read ‘That thin red streak topped with a line of steel’. In an alteration of the phrase, the police are sometimes referred to as the thin blue line.
See also thin as Banbury cheese, on thin ice, as thin as a rake.