The passive anti-theft system (PATS) is installed on every Ford Expedition at the factory. While some erroneously call this system the factory alarm, it really is not an alarm system. The PATS system will arm itself if someone attempts to start the vehicle with anything other than the PATS key programmed to that specific vehicle. Once the system is triggered, the red light on the dash will begin flashing faster than normal and it will remain in this mode until either the proper key is inserted and turned in the ignition or 15 minutes has passed.
Get into the Expedition and insert the known correct key into the ignition and turn it. If you use a non PATS key, such as one that was cut simply to open the doors from an all-metal key blank, the system will remain armed. If the Expedition starts, then you have disarmed the PATS system. If not, move on to step 2.