Can't close database

I have several databases open in DevonThink. Two have a dot showing to the right-hand side of their listing under Open Databases in the navigation pane. For those two, the “Close Database” command is not available. I did look in the documentation (and here) for help on this without success. What am I doing wrong?

The dot indicates unsynced changes in the database(s).
Is anything reported in Window > Log?