Cindy,  I have had that question quite a few times from various mothers.  These mothers all went through a number of tests and nothing was found.  That doesn't mean that a mother shouldn't have it investigated because there can be a number of health problems or medications that can cause this problem.  But if you need a reference for this to give the mother reassurance:
 
Barker, Burton, Lieve,eds.  Principles of Ambulatory Medicine. Williams & Wilkins, 1982, p. 792  

It is  not considered unusal for a mother to produce a few drops of milk years after her child is weaned.  Sometimes a mother is so concerned about the drops of milk that she continues to stimulate a milk supply by handling her nipples/breasts.  

Hope this helps. Valerie W. McClain, IBCLC

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