I am just guessing on this one.. I have never seen this before, and I am pretty sure I don't have this in my vehicle.. I would call the dealership that you purchased it from and ask them if they use any kind of ANIT THEFT/ NO Start systems.
Some dealerships used to use a system called Pro-lock or Pass Key. It was wired into the starting system and you had to have a special Computer chip on a key chain inserted in order for the vehicle to start. They would add these systems to the vehicles and offer to sell it to you when you went through final paperwork on the car. Sometimes it was easier for the dealership to just leave the hardware and wiring harness installed in the vehicle and just break off the computer chip into the reciever rather then to actually remove the system if if you did not want to purchase it. So you would then have a plug mounted under you dashboard that served no purpose.
So now back to your car, I wonder if that is they same type of system but uses a proximity sensor. Basically the dealership sales people have another key fob with them, that activates the system allowing the vehicle to be started. No key fob no start... If you don't purchase this from the dealership, then they just leave the hardware and disable the system.... This is just my guess. Since you purchased it used, it maybe from the original selling dealership to.
It looks like an alarm piece, like a bypass switch. Is it a push button? As far as the Pass-Key, that is built into a car from the factory, and is not an option, car has it, or it doesn't. There is a module used to bypass the pass key for an aftermarket remote start, but that is usually just a little box for GM. I know Ford products are different, and you actually need to leave a key in the module. You should have some aftermarket remotes. :blob:
No push button; just a plug, light and a few wires coming out of the back; I just noticed the flashing light the other day after the vehicle was locked/armed so I assumed this was a dealer installed security piece.