In Lion and Mountain Lion, you should be able to force xProtect to check for updates by running either
sudo /usr/libexec/XProtectUpdater
or
sudo launchctl start com.apple.xprotectupdater
I recently received this error on an ACAS scan on a new OS X Mavericks machine, but the target files for the commands above do not exist. I suspect OS X Mavericks is using something other than xProtect.