Yes, the database contains many groups; also some smart groups, some of which came bundled and some I created. Some of these smart groups developed different coloured icons as I mentioned elsewhere.
I’m running macOS 10.14.6 (“Mojave”).
This has happened before, once or twice (not sure if it was to the same database). I right-click to close the database, then open it again, and the disclosure triangle reappears.