OK, this is not a coincidence: for the third time this week Avast pops itself up after having done a Smart Scan hours earlier. But... it does so at exactly 12 hours of uptime on the PC! Almost to the second!
So .. if I run a Smart Scan after boot, it'll pop up the UI after 12 hours. If I run the scan 4 hours after boot, it'll pop up about 8 hours later. But at 12 hours of uptime .. bingo.
And yes ... this is still with a Smart Scan that fails, and I then click resolve all, I go back to the main window, and close it. Then hours later .. poof .. the Smart Scan results window pops up. With now no errors found. Also it clearly hasn't actually ran another scan.
In short: I run a Smart Scan. It tells me it found errors and failed. Then at 12 hours uptime, it gives me a delayed 'no viruses found' notice. The one that should show right after the scan.