I have just updated to 10.9.2 and after the installation reboot this little message shows up:
What exactly is happening in the background here?
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Sign up to join this communityHere's how I would determine what's running when you see messages like that:
It's Spotlight reindexing. The same notification appears after OS X 10.8.x → 10.9 update, as well as after all 10.9.x updates so far.