I seem to see the Private Topic folder highlighted erratically.

I am a member of a few, but the folder highlights (signifying a new post) when none of my topics have been posted to in days.

Is this picking up on "other" private topics that have been updated?

Yeah i noticed this as well, I keep thinking theres an update only to find that nothing's changed

I have just learned to ignore it since I don't have any topics in there anyways.