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?

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
Add image file
Upload is a website by car enthusiasts for car enthusiasts. It is not affiliated with any of the car or spare part manufacturers or car dealers discussed here. All logos and trade names are the property of their respective owners.