When one tries to decrypt a file which is not encrypted to any of the keys in their keyring, they get the message:
"Decryption failed: No secret key" and on the next line e.t. "One unknown recipient" or "Recipient: Some UID":
The Audit log shows:
gpg: verschlüsselt mit RSA Schlüssel, ID CD573B2B0736643A gpg: Entschlüsselung fehlgeschlagen: Kein geheimer Schlüssel
Edited on 2024-09-24:
As some users seem not to understand this, we want to add some text in a new line on the condition that the error message is "No secret key":
"This means that the data you want to decrypt was not encrypted to any of your secret keys."
And then be a bit more verbose on the recipient line, too.
Replace "Unknown recipient" with "The data is encrypted to: unknown recipient." or "The data is encrypted to an unknown recipient."
Choose the possibility here which works best in all cases/is easier to implement.
(I'm not sure what other possibilities have to be kept in mind here, I assume the "unknown recipient" is a variable and there are various other possible values.)