photos missing from photo book

I have recently opened iPhoto and have gone to do another photo book/Project, and was just checking an older book I made and have noticed some photos are missing from the book/Project? They are also not in the Album with all the photos I used for the book? Does anyone have any idea where these could be? Could they have been removed from the iCloud? I do not remember deleting these images from iPhoto so is there any way of retreiving them from anywhere else?

Many thanks, Ian