This notification means someone or human generated bot was looking for a page, file or any resource that doesn't exist on your website.
Why it happened?
Some themes or plugins are not properly wrote, so some files are missing that's why when a request to the item will generate such warning. If you're using such the theme or plugin, you may get the issue. I have several websites, one frequently gets this issue because of the theme and plugin it's using.
It not always be hacking attempts. If you deleted some files or pages, you may receive this warning too. Google, Bing, etc will come back to check for the file/page to make sure it still exist. Based on my experience, Google will come back for it last for some months after I have deleted a page.
It may also happen because of hacking attempts. The hacker generates bots that will automatically searching for files on your website for looking for weakness that may exist in your theme or plugins you're using. These are some samples that often appear on my site:
From above you can see they're trying access upload-file.php and thumb.php. These files are known to have problem and can be used for hackers to perform certain things that may harm your website. Luckily I don't use Mantra theme nor Modest theme. If you're using those theme you should update it immediately, because it will probably fix on the new release.
Sometimes, the problem can happen if you're visited buy Apple's visitors (iPhone, iPad, etc). Apple create new standard for improving user experience by looking some icons file. This new standard is not very well-known for most webdesigners. If on your View Log report, you see apple-touch-icon-xx, it means your site was visited by Apple's users. This thread has more information: