https://github.com/docker/docker-credential-helpers/blob/master/osxkeychain/osxkeychain_darwin.go contains the hardcoded string The specified item could not be found in the keychain. supposedly returned by SecCopyErrorMessageString (https://github.com/docker/docker-credential-helpers/blob/master/osxkeychain/osxkeychain_darwin.c#L7-L15) when an entry cannot be found in the keychain. Apparently that error message changed to No stored credential for XXX, according to the error messages reported in docker/docker-py#2482.

It would probably better to not rely on magic strings returned by SecCopyErrorMessageString, but instead (also) using the enum values (https://developer.apple.com/documentation/security/1542001-security_framework_result_codes?language=objc), probably this one: errSecItemNotFound.