Thanks for posting!
Your calculation is correct, so I suspect that it's storing the result. To change this, go to File-> Manage-> Database and click the options button for your calculation field. In the lower right hand corner, you should see a button named Storage Options that you want to select. Once you're in there, check the check box that says "Do not store calculation results -- recalculate when needed". This should resolve your issue and show "Search Mode" in that field whenever you're in a found set.
Works a treat!