If you are on iOS 5 you can go to Settings > General > Usage and see the list of downloaded apps, and how much space they take.
Also, try a restart or reset. Shy icons can sometimes be flushed that way.
Restart: Hold down the power button until the power off slider appears. Slide it off. When the iPad finishes shutting down, hold down the power button again until the Apple logo appears. Apple recommends you restart the iPad at least once a month.
Reset: Hold down the power and home buttons until the Apple logo appears. The iPad will reboot. This usually takes a bit longer than a restart.