This post is describe how to handle problem with "Missing Site Features".
You received for example such message inside PreUpgrade Report.
The following feature(s) are referenced by the content, but they are not
installed on the web server
Name = Unknown, Feature id = 0fa6a8ce-3f64-4d44-8419-69942b3e0471,Reference count = 3, Scope = Web, Status = Missing
You need run stsadm.exe command for locate where this site features is used.
cd C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\12\BIN
stsadm -o enumallwebs -includefeatures > c:\migration\features.txt
this command create text file that contain all site references for site feature in your
Sharepoint Enviroment, but isn't enough from this file you can't handle missing feature.
For this purpose you can use SharePoint Feature Administration Tool,
that can download for codeplex site.
This is very helpful tool for management and clean up site features.
You can find all missing features and fix problem in your SharePoint Farm by pressing
"Find Faulty Feature in Farm" or select relevant "Web Application", "Site Collection"
and "Web Site" that you can investigate from created early text file. Then you can
locate this missing site feature and uninstall it. Proceed the same action for all
missing site features you can resolve this issues.