Nobody has discovered or announced that there are Easter eggs in the game, i.e. hidden from the content user.
So I will reveal the first secrets
#1 Each season has its own individual musical setting (as well as the jingle announcing the change of the season every 15 levels).
You don't have to wait, the Ctrl+1 to Ctrl+4 keys will trigger the appropriate music
#2 There are not four songs, but ten.
The next ones are run by Ctrl+5 to Ctrl+0