Idiom: Read between the lines
OED definition: Look for or discover a meaning that is implied rather than explicitly stated
Example: Reading between the lines, I think he likes me
This idiom comes from the world of cryptography, which is the art of writing or solving codes. One way in which secret messages were hidden was by placing them between the lines of a different text. Although this technique had been used for some time beforehand, the expression originated in the mid 19th century. Despite it originally being used to refer to messages within written texts, it usage quickly expanded to refer to all types of unclear verbal and non-verbal communication. The New York Times contained the first print example of this phrase in 1862, and it has since continued to be popular.