This morning while reviewing new updates on my reposado server I saw this new update:
091-76348 macOS High Sierra 2018-04-10 
I didn’t think much of it; various “Install macOS High Sierra” updates have appeared in the softwareupdate catalogs since early in the High Sierra beta cycle: the App Store, when installing the “Install macOS High Sierra” application, downloads resources from these catalogs. (See https://managingosx.wordpress.com/2017/09/26/some-stuff-about-install-macos-high-sierra-app/ for more info).
But then I saw this cry for help on the munki-discuss list: https://groups.google.com/forum/?utm_medium=email&utm_source=footer#!msg/munki-discuss/I9nA-340mO4/KVQTJMEGCgAJ
Apologies if this has been asked and answered already, but we’re in a desperate time crunch. This morning, on the second day of standardized testing for our district, High Sierra is appearing as a “regular update” instead of an App Store option, so naturally MSC offers it:
It appeared that “macOS High Sierra” was being offered as an Apple software update (which Munki was then offering to install).