By transaction log do you mean PhabricatorUserLog ?

Oh, no -- on the passphrase object, it says stuff like:

epriestley created this credential.
epriestley updated the title from "X" to "Y".

...using ApplicationTransactions. The idea is to apply a new type of "edit" when you look at the secret, which doesn't change anything but just adds a transaction to the log:

epriestley examined the secret plaintext for this credential.

So roughly:

• Define PassphraseCredentialTransaction::TYPE_LOOKEDATSECRET.
• Add Editor support (e.g., the transaction always has an effect, but makes no changes).
• In the dialog workflow before we show the secret, apply a transaction "edit".