On Attica Dominatus, you can find a rather out-of-place Christmas setting in a hidden room.
From the start of the level, drive forward and do the first two small jumps, then rest your tire over the pipe on the top of the jump. Back up a tiny bit, then hit the gas and hold back on the control stick so that you flip onto the previous platform.
Reverse all the way to the back of the room and you'll see that the entire wall has fallen over, creating a new path. Reverse through this path and once you reach a certain spot, the floor will collapse and you'll be dropped into the secret room.
It doesn't look like this egg holds any kind of significant meaning other than to be festive, considering the Big Thrills' December 1st release date.
Confirmed irrelevant to the Trials Riddle by ANBA on May 27th, 2011.
"Christmas scene is not part of secret."