Launch agent that notifies when XProtect updates. Drop this into ~/Library/LaunchAgents and log out and back in or run: launchctl load ~/Library/LaunchAgents/xprotect_watch.plist

xprotect_watch.plist

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
  <key>Label</key>
  <string>fi.iki.sti.xprotect_watch</string>
  <key>ProgramArguments</key>
  <array>
    <string>/usr/local/bin/growlnotify</string>
    <string>--title</string>
    <string>XProtect Update</string>
    <string>--message</string>
    <string>XProtect.plist was updated</string>
    <string>--sticky</string>
  </array>
  <key>WatchPaths</key>
  <array>
    <string>/System/Library/CoreServices/CoreTypes.bundle/Contents/Resources/XProtect.plist</string>
  </array>
</dict>
</plist>