Near the beginning of the movie, Tommy, Frankie, and a third guy are stealing a safe from somewhere. Tommy instructs Frankie to sing "Silhouettes".
I initially thought that Frankie was a lookout and that was their signal that there's a cop coming and to either a) abort and get out of there or b) stop moving the safe and making noise and continue once the cop leaves.
However, when Officer Mike comes along, Frankie sings the song, gives his BS explanation, and then we see Frankie running to the car.
What was the point of him singing if it wasn't a signal for anything?