Consider the offstage trumpet. Many composers have used it — Mahler, Respighi, Verdi — but the piece I remember it in most is Beethoven's Leonore Overture Number 3. I was buried in the string bass section, still learning to play the instrument, while Jim Reed, first-chair trumpet of the Central Kentucky Youth Symphony Orchestra, stood in the wings of Memorial Hall blaring the call.
But it could be any orchestra anywhere, the trumpet in the distance, like the call to hunt or the approach of a royal entourage. It's the acoustic equivalent of painterly perspective, a tonal shading, extending the orchestra beyond the stage.
Hearing it played (from minutes 9:17 to 10:12 of this recording) makes me think something important is about to happen. Not here, of course, but somewhere else. It is, therefore, a reminder to pay attention to the faraway and forgotten, to what's offstage as well as on.