macos – How to diagnose a dialog box that flashes up unpredictably, at irregular intervals, too fast to read contents?

Over the last couple of days, I’ve repeatedly had a dialog box appear for a fraction of a second and then disappear, much too quickly to read the contents. It’s happened maybe 4 or 5 times so far; it comes unexpectedly, not apparently prompted by any action I’ve taken; I’ve been in Safari each time it happened, but on different websites, and it’s seemed more like another application momentarily stealing focus rather something from Safari or an open website (e.g. it appears centered on the whole screen, not on the Safari window); I spend much of my time working in webapps, so being in Safari just by chance each time isn’t unexpected. It’s occurred on two different devices (home laptop and office desktop), running different Mac OS versions (Ventura 13.6.1 and Sonoma 14.1.1), but under the same Apple ID.

How can I even begin to diagnose this? As I understand from Googling around, such flash-up dialogue boxes can sometimes be caused by malware, but equally by legitimate software (e.g. auto-updaters for installed applications). I’ve done the most basic safeguards against malware (all OS security updates installed; checked that all listed login items + background items appear legitimate), and found no red flags. But I’d really like to know what’s going on! Is there a log file where I can check through recently-appeared windows/dialogs, active apps, etc, or anything like that?