Is it a button? Could have been for anything. Goofy neon lights are the first things to come to mind, just as an example. Nitrous?... (really though) Hopefully not a hidden compartment. Do you hear or see anything when you hit the button?
I'd guess it has to do with a modification of some sort. Make sure to check the engine bay for anything else that could be suspicious in a more problematic way. If they left this mystery behind they could have left other unwanted parts behind, as well.
If it's not a button at all then disregard.
EDIT: Is it a lockout? As in if you press the button can you take the key out of the ignition with the car still running?
Lol. It looks like a pushable button but I'm afraid to push it!
Might be a ejecto seato cuzo! Or self destruct!
Guess I'm gonna find out today. Wonder if I can get the wife to sit in seat while I try it?!
Looks like its tapped into the main wire cluster under dash like a alarm. But I found no siren or other signs of aftermarket parts. I think it may be a tracking system doe theft but dont know why it would have a button.
Gonna go get snow off of car and see what it does.
Seriously, though. An engine lockout would be my guess. Press the button and try to take your key out of the ignition. If the car keeps running that means it's a lockout. Also try holding the button while taking the key out. Put the key back in, turn it to running more (words?) and you should be able to drive away. Hit the gas or try to put the car in gear without the key in the ignition and it should die.
You could also go to a shop that installs keyless entry, alarms, and lockouts and ask them. They should be able to tell you right away. It's a pretty low key setup, which leads me to believe it's one of the aforementioned professionally installed setups. If it were for a boy racer product it would probably be mounted on the a-pillar with NOS or Monster Engergy stickers on it.