When someone tries to duplicate a copyrighted tape, a hidden signal makes the recorded picture fade in and out, while the soundtrack remains steady.

She did not acknowledge, but by some hidden signal the boy knew she had received his message.

Make your lonely hearts ad and reveal the hidden signals of attraction.

Clyde Burke, who was lounging on the sidewalk, received a hidden signal from Moe.

"How confident are you that they'll know there's a hidden signal?"

Unobtrusively and as if by some hidden signal, the others had returned to their work.

They knew that Darius was keying the answers with some hidden signal only the horse understood.

While he was asking questions, Doris and Lexie were passing hidden signals, all with the intention of keeping him in the dark.

The guard at the door, obeying some hidden signal, snatched the rifle from his fingers.

"Come," one of our guards said, answering some hidden signal.