Answer:
This is likely an example of a referential delusion.
Explanation:
Schizophrenia is a mental disorder characterized primarily by its hallucinations and delusions.
In this case, Sally believes there's a special meaning for her in a billboard. This is known as a referential delusion, which is characterized by an individual believing that something from the environment is directed specifically for them, such as seeing hidden messages in the television, for example.