Even opletten geblazen.

https://512pixels.net/2017/11/apple-begins-high-sierra-automatic-rollout/

http://tidbits.com/article/17633

Apple Starts Pushing High Sierra on Unsuspecting Mac Users

If you’re running macOS 10.12 Sierra or earlier, and do not want to upgrade to 10.13 High Sierra right now, be careful because Apple has started pushing High Sierra to older Macs and making it all too easy to upgrade inadvertently. In short, if you get a macOS notification asking you to install High Sierra, click the Details button to launch the App Store app, and then quit it.

Ik had het ook gezien. Hij is van de downloads verhuisd naar de updates, als een gratis upgrade.