Have you checked My Devices in your Google Account?
I have an "unknown" listed on my Google Account. Which was actually a Bluestacks session running on a Mac, last used in March 2018. As long as they're Signed out, it's OK I think. View attachment 163125
Do the Sign out for it, if you haven't already done so. If you've already changed your passwords, also enable two factor authentication(2FA) login to Google, like using the Google Authenticator app or SMS verification. Then any devices won't be able to login automatically, and will need a 2FA code.