From what the OP has described, it sounds like a function of Google Chrome running your username and password through a database of known compromised credentials. It's hard to be certain but it sounds like it.
https://support.google.com/chrome/thread/23534509?hl=en
the way the scan works if is that it is not specific to the site you're are logging into, just that your username and password combination has been compromised sometime someplace. This shows why it is a very bad idea to use the same password on multiple websites, once your creds are compromised in one place they can log in elsewhere.