If you’re not sure what security only crawl is, please see some of my earlier post on this subject. It’s much easier to troubleshoot security only crawls now that we have included Security Updates category within the crawl log in SharePoint 2010! For example, now I can easily spot the incremental crawl at 11:03 AM was a security only crawl:

Just wanted to point this out and provide a heads-up that some of the advanced troubleshooting techniques from the past can now be avoided.   Thanks Colin for supplying the pic.

-Russ Maxwell, MSFT