theft light blinks 1999 Windstar

When my wife starts up her 1999 Windstar, the theft light blinks rapidly for a few minutes, then it slows down and gets intermittent, and finally
, after about 10 minutes stops blinking on and off. We don't have any additional security systems, just the keys with the chip in 'em. The theft light is the one that normally flashes when the van is turned off.
Any ideas?
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