2001 F150 Theft light

Today my Theft light just came on after stopping at a parking lot. The light keeps flashing. I check the owners manual and it states that the
theft system be's armed when key is in off position, it also states that if the light goes off while the key is in the on position there is no problem with the system, which my light does go off. I made sure that the steering wheel is locked. What could be wrong. It is still flashing since four hours ago. ps. I am using the real ford key
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