Skid Mark is one funny dude...
Sounds like you are playing the Piano.
That's correct. I believe there's more stuff on Trial mountain. Coming down the hill past sector 3, look to the background to see 2 people in a row-boat in the lake.
Here are the pictures, and I noticed something else:
Nessie:
http://img703.imageshack.us/img703/2167/trialmountaincircuit3z.jpg
Two people on a boat
http://img703.imageshack.us/img703/2167/trialmountaincircuit3z.jpg
It's hard to ignore the fact that Nessie was at the other side of the lake and the people on it in the middle, but that she is still more detailed then the people, so it was actually meant to be there and to be found. I'm still wondering why you don't get any trophy for it...
Who's Nessie?