In the Hardy Boys book “Hostages of Hate”, Frank Hardy’s girlfriend, Callie Shaw, is kidnapped and is able to send a secret message to Frank by blinking her eyes on a video recording.
There is a Hardy Boys book in which one of the boys makes a phone call to family with some nefarious person present. He holds an innocent conversation while sending a message in Morse code by tapping the phone with a pencil.
In the Hardy Boys book “Hostages of Hate”, Frank Hardy’s girlfriend, Callie Shaw, is kidnapped and is able to send a secret message to Frank by blinking her eyes on a video recording.
Funny what sticks in your mind from a young age.