Apple Watch Series 4 (44mm) Random Chime Sound

As the title shows, I have an Apple Watch Series 4 (44mm), with LTE capability.

I've been getting a random tri-tone (three chimes ascending very quickly) chime on the watch. The oddest thing about it is that there is never any notification that goes with it, and it doesn't come in any apparent time pattern. It's not hourly, it's just very random. I'd say it happens a minimum of once a day, usually more.

I have haptic alerts on, and the watch isn't in silent mode when it happens. I do have the notifications indicator turned on, so if it was an alert I should see at least the red dot at the top of the watch face and then the notification, but there's never any notification, just the quick sound.

I've seen a similar problem discussed in an outside Mac forum on Series 3 Watches, but it never happened to me with my Series 3 watch.

What could this be?

Thanks!