In my innocence I clicked on the Carousel icon in Dropbox. Who knew that would activate Carousel?
I don't and didn't want it activated: I was just taking a look at what it is.
I asked Support how to deactivate it and they explained that I needed to go Carousel and then log out. That, according to Support would deactivate Carousel and it would stay inactive as long as I didn't click on a link to Carousel in Dropbox or visit carousel.dropbox.com.
What a journey around the houses.