I found a notation on a VotV wiki that said:
Occasionally, a Signal will sell for 0 Points despite being a unique Signal with a unique ID associated with it. Make sure to refer to the Signal Glossary in-game or the Signals page to confirm if a Signal is a waste of time to sell off.
The Glossary can be accessed by entering the Pause menu, selectingStats, then selecting theGlossarytab at the top of the new window.
I have never encountered that message, so I am not sure if it will help you, or is even relevant..