Based on a web game called "Find the invisible cow"
An invisible object is somewhere in the window. A very short sound file plays on a loop until you click on the object. The volume of the audio file is inversely proportional to the cursor's distance from the object.
Source code available under GPLv3, assets available under CC-BY-NC-SA 3.0 or 4.0.
See app's About window or Credits.rtf for more details. Full GPL license available in LICENSE.