Be an open secret
Definition 1 of 1
Pro Tip 1/3Implied Discretion
This phrase often suggests a situation where discretion was expected, but the secret became widely known. 
His resignation was an open secret long before the official announcement.
Pro Tip 2/3Sarcasm or Irony
The phrase may be used sarcastically to highlight the failed attempt at keeping something secret. 
Their romance was supposed to be hidden, yet it's an open secret.
Pro Tip 3/3Avoid Gossip
Use this phrase cautiously as it might imply involvement in gossip or spreading private information. 
You know it's an open secret about the manager's payoff, right?