I'm glad I'm not the only one having the problem. This Saturday was the first day I noticed it, and the fact that it seems to have started for both of us around the same time definitely makes me think whatever is causing it is the same for both our phones. I haven't received any over the air O/S updates that I know of, and I haven't changed any settings, which leads me to believe it's an app that is causing it. I wonder if there's a bug on a recently updated app that causes the notification sound to play but doesn't show an actual notification (and isn't tied to the notification settings within the app itself).
The problem is I usually update apps when I received the Google Play notification, so I can't remember which apps have had updates recently and I couldn't find a way to sort my apps in Play to show recently updated. If it is an app bug, hopefully the maker will figure it out and fix it. Keep me posted on what you find out and I'll do the same.