I've found some unusual activity in my site's Apache logs and every hit has this UserAgent string:
"Mozilla/5.0 (Windows; U; Windows NT 5.1; en-gb; SiteCon/8.8.14)"
What is SiteCon?
It turns out the UserAgent is attached when using the web site monitoring / uptime provider Site Confidence: http://www.siteconfidence.com.au/
Remember that user agents are free to send any header they want to (even none). Lookup the IP addresses associated with these user agents. If they're all much different, it's likely a bot network of hacked machines which use a script that identifies itself with "SiteCon".
Aside, "SiteCon" sounds like "Site Connection" or similar. Perhaps it's a flood tool?