Just looking through the error logs of some of my domains: I've learned a bit about the crossdomain xml file and how it's used for flash files, but I don't host any flash files and I was wondering why anyone / bot would go looking for such a file. Are most of these entries just spammers / hackers checking for outdated scripts and install files to exploit ? Should I be worried ? Should I upload a blank crossdomain file just to stop registering 404's ?
If you're not using it I would upload a blank crossdomain.xml (this is if you just want to get rid of the errors, there's really no other purpose). There could be many different things trying to access that page; from a plugin on your site to google's spiders. It's hard to tell with the information given here.
If you use Adsense on your site (or any other advertising network for that matter that shows dynamic ads), you will see these browser requests to crossdomain.xml to check whether they should trust to play those Flash ads from 3rd party domains. The best solution would be create that file as required. The blank file might disallow playing those ads (thus reduce your revenue from advertising)