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The gas button is black. I would guess it's a switch for one of those add-on do-hickies. Some people just can't drive one without a thing-a-ma-jig of some kind.
You'll prolly have to trace the wires and see what it goes to. My previous C5 had a similar button that went to air horns!
Sounds like something the previous owner installed. It is nothing factory installed. If you can contact the previous owner in any way they'll be able to tell you. It could be any number of things like after market alarm, line lock, etc.
Otherwise you may be faced with chasing the wires to see where they go which means taking apart the console.
I like the easy button. I was going about 80 MPH and decided to try the button. Well holy crap, the thing took off like a rocket, reached 120 in about 3 seconds. I couldn't believe what had happened, so I slowed down to 40, hit the button again. Well the tires let lose, snaking and squeeling down the highway leaving rubber the whole way. Now I was real curious, so I pulled over, looked in the trunk and found two blue bottles mounted in the center compartment. They said NOS on them, I wasn't sure what this was. So I drove to a performance shop and had the guy come out and look. He said well you have two stage Nitrous, a supercharger and a modified ZO6 engine, probally putting out 700 HP.
Then I woke up, found the button was for the amp in trunk. Oh well, dare to dream.