" . . . that pane told me that no updates were available but it also did not list updates installed in the last 30 days (which it did before) . . . "
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Not sure what is up for you, Jon.
I get the APP store "little red number in the blue circle dock icon", as before SIERRA, stating how many updates are available (both Apple and 3rd-party apps). I go to the APP STORE using the Apple symbol in the top left hand corner and select Updates. The wheel spins and then the new updates all appear - matching the quantity as was indicated on the little blue App Store icon in the dock. I install each one . . . one at a time . . . and then each one, in turn, appears listed below as "installed within the last 30 days."
In a nutshell: The behaviour is exactly the same as it was in EL CAPITAN.
The only thing which does not appear in the "Installed in the last 30 days" panel, per se, is any of the SIERRA system updates which have been installed (i.e. 10.12.1). However, any Sierra Apps (e.g. Safari, iTunes, iMovie, etc.) - as well as third-party apps - all show up in the installed list immediately after installing.
Again, all appears to be the same process as with El Capitan.
FYI : I have my System Preferences for the App Store set to only notify me of any updates. I have not activated the Automatically Install Option for any downloads from the App Store.
P.S. West Coast Canada here. Not sure if this info helps, but thought to chime in anyway.
