In today's Sun we have proof of aliens invading London!
The Sun claims that some guy was taking photographs in Covent Garden and after getting the pictures developed one of them showed "flying saucers".
Oddly the guy didn't notice them at the time, they only appear in one picture and were not spotted unti after he had them developed by Boots and he showed them to his wife.
The most likely reason is that something went wrong in the development process.
Of course, as the numerous comments on the Sun say, it could be a reflection if they were taken behind glass.