Have you been monitoring the coolant level and know how much coolant is being lost per week or per miles?
With 145,000 miles, i would be looking at the hoses, water pump, coolant reservoir and radiator for signs of weeping coolant ie; very minor seepage. Have you looked at the floor or ground where you park your fiesta most often at work and home? Not knowing what maintenance has been done in the last three years, any of the things I mentioned could be the culprit.
These kinds of situations are impossible for anyone to exactly diagnose on a website. But, there are threads on this site that have discussion of the problems of that area.