audio – What is causing the “complete” notifcation sound, “doo-doo-deet”?

I have a new issue I’m perplexed by. Even with the Mac (2015 MBP 13) on screensaver, about once an hour or so (not sure how long TBH), I hear it make the “complete” sound notification, like something is “done”. It’s the familiar “doo-doo-deet” happy bell like sound used for a lot of things on the Mac. I’m on Big Sur, and this only started happening this week. I haven’t installed anything new. I can’t seem to figure out what is doing it. There are no notifications or traces of evidence I know how to find to tell me what is going on. Notification center only had a few things in it that were from previous days, nothing in the last hour from when I hear it last.

Any ideas? Definitely scratching my head.