If you had Chrome installed before but you just weren't using it, possibly it decided to silently auto-update in the background one day.
Another possibility is what BBJ mentioned, some other application installer or updater that offers Chrome as a bundle somehow installed it.
If you decide to investigate further, you can check the modified time on the main Chrome executable and the installer log located under C:\Windows\Temp\chrome_installer.log (or possibly under %appdata%\Local\Temp) if it's still there.
Might I just add that Chrome, especially if you are signed in with a Google account, provides a lot data to Google (ostensibly, mostly "analytics"). Should you be concerned with privacy, Firefox with uBlock Origin (as an ad...