Saucer shaped object appeared in photograph

A saucer shaped object appeared in photograph but only when looking back at the pictures. The photograph was taken when were at the top of Vidova Gora in Brac Croatia, taking in the view. My husband and I were the only two on top of the mountain at this time. There was no noise and was not visible when taking the photograph.

I cannot describe it as it wasn't there until looking back at the pictures. I took many pictures in succession and this is the only picture it appears. Therefore I cannot believe there was anything on the lens either.