What are the reasons for images going missing without changes made? I have a client who is complaining that images which were there are no longer there. I wasn't sure what to think about it, so I asked him to add them again and let me know if it happens again, meanwhile I took note of the file path (/sites/default/files/styles/large/public/map.jpg).

I have now received another email saying they have disappeared, and upon checking the default Drupal broken image can be found within the WYSIWYG, but there is nothing on the actual page display of the node. I checked the file path and the image still exists on the system. There is also no new node revisions, so they have obviously not messed around with it.

Could anyone provide me with a list of explanations on what could possibly have happened here? I'm confused and I have no idea where to start with this problem!

FYI Drupal version is 7.12!
There are known issues with the images after the security update of Drupal 7.20. I would suggest you to update the latest release if it is the case.
I would also check if there is a .htaccess file preventing access to the files or folders. I remember that there was a similar issue and it was related with a .htaccess file restricting access to the files under the path where the images are saved.