So far, as you can see from the image above I've found 4, and if you're reading this you want to find them too, so read on!
In Greenhorns Grove I have found two so far;
On Waterworks, when you get to the section where
this platform falls forward, you will need to reverse up it and carefully down the ladder til the circular sign is off your screen, this puts you perilously close to dropping off the ladder so be careful! Once you've done that, you need to get back up the ladder and as you will see the
sign has changed. Head for the end of the level and bail-out over the finish for your cutscene.
On Observatory, head along the track as normal til you get to the top of the red ramp used as a challenge where you'll see
this sign, drive past it til it's off screen and you hear a sound effect (but not too far onto the raising platform) then reverse back past the
changed sign and all the way back down the hill to the bottom.
In Urban Sprawl I also have two;
On Skyscraper Showdown simply get to the end of the track and bail-out over the finish, no screenshots needed for this one.
On Park and Ride, when you get to the
platform that raises you'll notice that under it is a squirrel sign, crash or bail-out to
land in the box marked by the sign and wait.
So far that's all I have, but if anyone else has more to add, feel free to post (with spoiler tags) where you found them, screenshots and/or videos to help out with locations are welcome.