Ejector button?

 

One quick method to test for an aftermarket alarm is to disconnect the battery for a few seconds. When you reconnect it it will likely start screaming if you have one. Then, push the little black button to put it in valet mode and disable the alarm.

 

I have a similar set up on my F150 but that is for an added alarm from the dealer. I'm sure that is what you have there. I also have an Explorer with a similar switch, and it is for the auto start/alarm, so it would be a good bet that's what yours is for.

 

I have something similar and was told by my dealership when I bought my '11 that it was ADDED (not Factory Ford) as a additional anti-theft used by dealerships. They actually asked me if I wanted to pay more and they would activate it (as it was deactivated when I bought the car) for a added mesure of protection.

Matthew