The Men's Shed iMac had an external HD with an experimental basic installation of Sierra on it. I fired it up just to see how many updates were available, it said 15, including Microsoft OneNote & OneDrive, neither of which are installed on the computer or the ext HD.
I'll admit it had me a bit flummoxed, then I realised the USB stick which has the Sierra 10.2.2 combo updater had copies of those two apps on it & the MAS was obviously seeing those & wanting to update them.
Not happening, I decided. I rebooted into the Yosemite installation & repartitioned the ext HD to remove the Sierra install, allowing the freed space to revert to Time Machine duties.
I'm now installing Sierra on the iMac itself.