M4 iMac running the most current version of Sequoia. Something weird just started happening. At shutdown right before the screen turns off, I mean like a tenth of a second before, a small dialog box appears in the upper left corner. I cannot read anything it says since it appears *right* before the screen goes dark. There’s no time to even take a screenshot. The only thing I can discern is that there might be a Cancel button but I can’t be sure. Everything is working properly on the machine.
Is there any way to find out what this thing is trying to tell me? Is there any log and how would I find it?
Try logging out instead of shutdown. Does the dialog still appear?
Try starting a movie capture of that area of the screen. Maybe it will grab it. It depends on whether the Mac kills the capture before it happens.
There’s probably evidence of what’s going on in the Console log, but good luck finding it. OS X 10.12 Sierra destroyed the usability of the console log for regular users – now it is like finding a needle in a hay storm.
If you use an external monitor it might be flashing a loss of signal message (mine does in top right corner but it stays there for a couple of seconds).