My Blancpain Bathyscaphe was bought brand new, and the caseback was clean as can be, I know because I took many macro photos like yours. After a few months I noticed a quite large white spec of dust pop out of nowhere. It's frustrating and unfortunate, but dust can be hidden in there and eventually reveal itself down the road. No point sending the watch in just for a dust issue... so I've just learned to live with it.