To hear something through the grapevine is to learn about something. It can also simply refer to an overheard conversation or anonymous sources of information, e.g. "I heard through the grapevine that Brad died."
https://en.wikipedia.org/wiki/Grapevine_(gossip)
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hear (something) through the grapevine
To hear or learn of something through an informal means of communication, especially gossip.

hear something through the grapevine or hear something on the grapevine BRITISH
COMMON If you hear a piece of news through the grapevine or hear it on the grapevine, you are told it informally by someone who was told it by someone else. I heard through the grapevine that she was looking for work. She heard on the grapevine that he had come back to London. Note: One of the early telegraph systems in America was given the nickname `the grapevine telegraph' because the wires often became tangled, so that they reminded people of grapevines. During the American Civil War, the telegraph system was used to communicate propaganda and false information, as well as real news about the progress of battles, so that anything heard on the `grapevine' was likely to be unreliable.

https://idioms.thefreedictionary.com/hear through the grapev...