UFO Sighting in Dundee, Scotland on 2017-07-28 16:33:00 - Photos taken before and after even while looking at dolphins. object appears on one photograph in the middle of the set taken. not before or after.

I was at the beach with family and we saw dolphins in the river tay. my girlfriend from bulgaria doesn't believe that we have dolphins in the river in scotland so i tried to snap some shots of them with my sony xperia x 23mp camera. i took several pictures while focusing on the water and when i looked back to see if i had caught any dolphins emerging, one of the photographers in the middle of the set taken had a strange brown egg shaped object in the sky. clearly the object is in the distance and is not dirt on or in my lense as my device is waterproof therefor airtight and nothing shows in the pictures before or after. i just can't explain it.