Had ours go off last week, and upon checking the pressures, one was down to 25psi.
Was curious to see if I had reset it properly, page 11 of this PDF seems to explain it accurately - it's definitely based off rolling circumference calculated by the
ABS sensor:
http://www.fordservicecontent.com/pu...N13FOCqg1e.pdf
Although I reset it while driving, so should remember to reset it tonight.