Had a pretty noticeable dust particle within the outer camera lens when I unboxed my XS. I always feel a little uneasy when returning stuff but I knew this would drive me crazy. Asked for a new phone saying that if the particle moves over the center of one of the lenses, it may distort the photos and affect the performance of the camera. They exchanged it without question.
What I’m saying is, it could’ve always been there and eventually made its way to more noticeable area. Don’t know how a dust particle of such noticeable size can enter from the outside.