I had another look at this.  I did not issue any command to make an asumption.  But at a later point, this variable a was in the integrand of a fairly complicated integral for which I believe the correct computation requires the introduction of abs(a) to account for two cases.  From this step on  the variable appeared with the "~".  So it looks like Maple is telling me it made an assumption at this point.  That sound right?