Fly On The Wall Meaning

(idiomatic) A quiet, non-participating, or unseen observer; an eavesdropper or witness.

Example: 1918, "Clark Would Draft Even Men of 68," New York Times, 9 Aug., p. 11 (retrieved 28 Oct. 2008),
  James W. Gerard, ex-Ambassador to Germany, the first speaker of the evening, said that at that moment he would like to be a fly on the wall of the palace at Potsdam to hear what the Potsdam gang were saying about our soldiers.