Tire Pressure Sensor?

I apologize if this is a dumb question, but I got new tires installed today. A family friend owns the place that installed them and I asked him to show me how the machine works that mounts the tires, then he told me you have to be careful when the whiles have tire pressure sensors. I told him i didn't think my car had that option (2004 ford explorer xlt 4.0l sohc) and he assured me it did. I'm just curious why do my wheels have tire pressure sensors but my dashboard doesn't have an option of a low tire pressure light? Or am I missing something, because I've had a completely flat tire and nothing has shown up. Once again I apoligize if this is a bad question, I'm not much of a mechanic. Thanks for any help.