blog[.]google/technology/safety-security/google-password-checkup-cross-account-protection/
... we want to provide you with the same data breach protections for your accounts, beyond just Google apps and sites. This is where the new Password Checkup Chrome extension can help. If we detect that a username and password on a site you use is one of over 4 billion credentials that we know have been compromised, the extension will trigger an automatic warning and suggest that you change your password.