If you hear something “through the grapevine” it means that you have heard a rumor about someone or something. If you use this expression, it also helps you protect the name/identity of the person who told you the rumor.
Have you heard something through the grapevine recently?
-David: I heard it through the grapevine that you are looking for another job. Is it true?
-Bob: Wow, how did you know that? I only told a few people. But yes, it’s true. Please don’t tell anyone else.
You can also say “I heard it on the grapevine.”
A similar idiom: “A little bird told me.”