I think a “secret” pardon would be disallowed when the contention reaches the USSC as being NOT a pardon at all. I don’t think there can be a secret pardon.
A promise/guarantee not to prosecute from the AG due to recusal is a practical pardon. Lois Lerner was pardoned.
She was pardoned when Comey did not charge her citing “she did not intend” to break the law. She gummed up the works by losing.
They can hide a lot of corruption under the ‘National Security’ blanket.................