"App Explorer Updated" on Startup

Whenever I migrate my files to any new pc, I get the attached screenshot showing up on startup - obviously one of my old files is infected with something, but no amount of scanning with any program I'm aware of has located any infection, nor do I see any suspicious startup entries anywhere. I've noticed the message content seems to be randomized (on some new PCs it'll say something like "Windows Startup Menu Updated" instead of "App Explorer Updated" for example.

Anyone run across this one before?

Thanks in advance!

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