Hey EGOL
I have seen this in the past on my own site and on a few client sites in the past (which is not to say I have an answer here).
We were seeing completely random looking URLs that at first made me think the site had been somehow hacked or compromised but further investigation revealed that was not the case. We were just getting the strangest of links to pages that did not exist like xhyx.php?id=jamesbrown (that kind of thing).
We did nothing here and over time it seems to have resolved itself and these pages are not listed any longer. I tend to think of the webmaster tools data as diagnostics and it is telling me these pages don't exist so I can check for problems. Well, there is no problem, they don't exist and I am happy about that. Still, whether to mark them as fixed or not, I am unsure and would err towards not doing anything with them as they are not 'errors' as far as I am concerned. Likewise, I don't want to redirect them in most cases as I don't like the linking sites and have better things to do with my working day (I am not getting that time back - it's the digital equivalent or ironing clothes or some such laborious grind).
I had a look around again and whilst I can't find any specific answers regarding whether to mark them as fixed the following posts are of interest:
- http://productforums.google.com/forum/#!topic/webmasters/3GTOLCE-8pk
- https://productforums.google.com/forum/?hl=en#!category-topic/webmasters/webmaster-tools/rKI-38ohfbc
Particularly this quote from John Mueller at Google (webmaster tools guy I believe):
"In general, if a URL is really a 404, that's fine for us, and not something that would cause your site any problems in the long run. At any rate, you don't need to "fix" this problem (eg with a 301 redirect), if you're sure that the URL should really not exist. Having 404s listed in Webmaster Tools will generally not affect your site's crawling, indexing, or ranking; it's normal for websites to return 404 for URLs that don't exist."
So, my take is not to bother but would be interesting to ask the question in webmaster tools section of the Google product forums: https://productforums.google.com/forum/?hl=en#!categories/webmasters/webmaster-tools
Not an answer as such but hope that helps.
Marcus