Secrets

Secrets are concealed information that, if uncovered, can potentially damage a noble's reputation. Ambitious nobles seek to uncover the secrets of others for leverage while taking steps to conceal their own.

Codex
"Some actions and events are only visible to the characters that are involved in them. A subset of these hidden events create secrets, i.e. some kind of evidence that can be used to show what happened, but that the characters would prefer to keep hidden, usually because they have committed some kind of crime and do not want to be caught.

Each turn there is a small chance that other characters that know you, or the security team of other houses, will discover one of your secrets. Some secrets are harder to discover than others, and you can see how hard a secret is to discover in the Secrets tab of your character. You can ask your Head of Security to cover up a secret for you, which will increase it's discovery difficulty. Gradually, most secrets tend to become harder to discover anyway, as the evidence to support they ever happened is erased by time. You can actively try to discover the secrets of another character (if they have any) by assigning a member of your security team to investigate them.

If you become aware of another character's secret you can reveal it, potentially damaging their political standing. If the secret was about a crime they committed, the victim will have a grievance against them when the secret is revealed. You could also use the secret as blackmail, by making a request someone would not otherwise accept."