The App Store Updates page on my mum's Macbook Air now shows that she has installed two updates called "Security Update 2017-001", and she started to worry that she didn't have the correct update installed today.
They both link to the same page (https://support.apple.com/en-gb/HT201222), so it took me a while to figure out what was going on, and that she really had installed two distinct security patches (one before upgrading to High Sierra, one after).