Bigs the Cat in Sonic Adventure 2

[Psychospacecow]

In Sonic Adventure 2, Bigs the Cat is hidden in a few missions.

In Sonic's first mission, he is just to the right when you're being chased by the GUN truck.
In the sewer mission for Knuckles, Bigs is underwater at the first water pool when there is water.
In Tail's mission on the highway, Bigs is holding onto the rafters of the bridge and can be seen when falling off the bridge mid-way through the mission.

I haven't done this before, so what'll it take?

Edit: I was playing the PC port so I am assuming that it was the dreamcast version.