Forum Moderators: phranque
If there is a useragent associated with other log entries then these are most likely spiders (probably of a malicious type as they seem not to have fetched and therefore are disreguarding robots.txt).
If you are not logging useragent then this could be text based browsing (such as lynx) or users who have configured their browser not to display images, or wget, or a million other things - your best bet would be using a more robust log format (replace your CustomLog with "CustomLog logs/access_log combined").