Creation of the Marauders Map. >>>> It's because the map doesn't rely on vision. It shows people's physical place, which is still possible to find if you are under the cloak. Death could not find people under the cloak because Death was relying on vision.
This is a perfect explanation. I never complain but it's an excellent explanation for why they have HP marathons so often. :P But they should show a marathon every weekend. I would push the button every weekend. Lemme work for ABC!