All app downloaded from Mac AppStore(MAS) contain a receipt inside their app bundle (example: appname.app/Contents/_MASReceipt/receipt). MAS will scan this file to check whether the new version is available.
Sometimes the pirated app receipt doesn't necessarily use the same name as the app.
For example: The app name could be 'Angry Birds' but inside the receipt it's 'Cut the Rope'. Hence MAS will search an update for 'Cut the Rope'
You can use this application (Find Any File) to search any receipt on your drive: http://apps.tempel.org/FindAnyFile/
- Launch 'Find Any File' and search for: _MASReceipt
- Go throught each result that you think could be the culprit, copy the receipt to your desktop as a backup, and after that delete the receipt inside _MASReceipt folder.
- Open MAS and let it scan for software update. If it's still showing the update for the problem apps, it means that you've deleted the wrong receipt. Copy back the receipt backup from your desktop to it's previous location.
- Repeat the process until you find the pirated receipt
Most of the answers can be found here: https://discussions.apple.com/thread/3622324