What is a word that describes a secret that passes on from a person to person?
Solution 1:
One technical phrase for it would be "single-strand chain"---I tell you a rumor and then you pass it along to another person, who then tells another, and on-and-on....
"single-strand-chains" are a kind of Grapevine Transmission Pattern" or "Informal Communication Network", a strategy sometimes used in the corporative world.
Solution 2:
I believe you may be referring to "open secret" i.e. a "secret" that isn't a secret at all because everyone knows about it?