The secret track hidden in the script on becausetheinter.net was discovered by /u/peepsie112 on reddit. Here is the post describing how they found it. The hidden file was an acapella that lined up with the last track on K A U A I. It was confirmed by a tweet from Childish Gambino.
Mr Duck and I are working on deadlines, it’s 2am. Mr Duck just showed me the latest figures on the video. We’ve hit 4 million guys. The big 4. Photo Every Day 1-6.5 is now my most viewed creation in existence. All of this is incredible. I don’t know how to express my gratitude enough through these little words: thank you.