Today's English idiom is dedicated to JLL 07
A grapevine is the long trailing branches of a vine that when not cultivated in fields grows around houses and in gardens.
This might create an environment where you might not be seen but could hear others talking. Another explanation is that the brances of the vine can trail for many meters.
So if you hear something through the grapevine, it's unofficial and through rumour and gossip.
Here's a great version of the motown classic of the same name.