Amidst the shined chrome and complicated machinery of Isabelle lies a shy squirrel waiting for you to drop by.
To find the squirrel, go through the track up until the point where you need to hop on the 2 upright vents before the finish line. Slowly drive off of the side and try to keep yourself against the wall when you fall. Touching any of the horizontal vents on the way down will set off an explosion.
When you reach the bottom, drive forward. The camera should pan in to the end piece of a vent located close to the wall, and a squirrel will poke its head out.
It's a squirrel, alright :)
Name of the track seems to refer ISABELLE that was 200+200 GeV proton-proton colliding beam particle accelerator partially built by the United States government at Brookhaven National Laboratory in Upton, New York, before it was cancelled in July, 1983. (http://en.wikipedia.org/wiki/ISABELLE)
Just looking track this seems pretty stong case.
What we can confirm is that it's a squirrel. Given that they are located in numerous tracks, they may not be as significant as other easter eggs.