SQUIRREL #1 - WATERWORKS
Once you land on the platform above the water that falls forward, stop and reverse back over it to find a ladder to the left-hand side.
Roll back down the ladder until your rear wheel touches the water, then ride back up it - you can see a squirrel sign has appeared on the side of the platform.
NOTE: As long as you reach the bottom of the ladder, you can still collect the squirrel if you crash and respawn at the checkpoint.
Complete the track and the squirrel will be revealed.
SQUIRREL #2 - OBSERVATORY
When you reach the top of the red-sided ramp, slow to a crawl and roll past the 'Slow Down' sign until a chime sounds.
Stop before the launch platform in front of you and go back down the track to see a squirrel sign has been revealed.
Keep going back down the hill until you reach the water, where you'll find the squirrel and a big pile of explosives have appeared.
SQUIRREL #3 - CLIFFHANGER
Once you've cleared the canyon towards the end of the track, stop on the next ramp made of rock and use it to roll back across the platform.
Fall backwards off the platform and lean back to push yourself onto the road below.
A chime will sound if you landed in the right place, then simply wait for a cutscene to start.
The next squirrel will arrive, then things get pretty dark...