Windows 10, Ryzen 5 1400 PC. I had shut down Slack to avoid distractions, and then got a notification that it was time for a reboot for Windows updates. After I was done with the task at hand, I rebooted the PC. During bootup I tapped DEL to get into the UEFI settings. While I was browsing the UEFI settings, a Slack notification played through my computer speakers. I was perplexed but thought it must have been my phone. A second later, I got a notification on my phone, and that's when I remembered that my PC and Phone Slack apps have different sounds.
"OK, maybe somehow W10 has booted while I'm sitting here in UEFI" so I exited UEFI settings, booted into W10, and... Slack isn't running.
I have to admit that if it didn't happen to me, I'd never believe a person who told me this story. But I can assure you it's true and I'm sure of the circumstances I've described.
So how on earth did I get a slack notification from my computer after I had shut Slack down and was in the middle of a reboot?