Right, thanks for the query. That's a useful one.
I was responding specifically to the Kate's question about changing visibility status of items. It seems turning things on and off is not logged either at all or in sufficient detail as far as I have seen.
Turning things like activity plugins on and off as an admin is not logged in the Configuration Changes report, though it can have a huge impact on a site ;). There's nothing in the database log table tracking that change. Try disabling, say, the Assignment mod and see what happens: people will go beserk but the log won't tell you that it ever happened.
On the other hand, if you change a course visibility in course settings or via the Manage cats/course tool, it does log that the course was "updated" by whom & when, but it won't tell you what the update actually updated or changed. That could be useful circumstantial evidence though, sort of like "Colonel Mustard in the Dining Room...." but no more.