Doesn't seem that unnatural to me to still be able to express a drop or so of
milk long after weaning has occurred.  When our horse came to us, she was
separated from a colt she had been nursing for 1 1/2 years.  They had been
left to pasture with no previous separation.  She suffering from
 malnutrition due to pasture only with no supplemental feed, needing to put
on 400-700 lbs.  Twelve years later, I could still express a stream of milk
from her udders.  IMHO, there is probably a wide range of normal, just as
wide as the physical, emotional, and intellectual difference in all of us.

Warmly,
Pat Lindsey, IBCLC