RiM;
Maybe the Evans hi glycol content product doesn't expand quite as much as the normal glycol/water mix, but if coolant was being lost out the overflow, I suggest your Alfa Rad was overfilled and did not have a sufficient volume for expansion...
Source:
http://www.sw-em.com/Cooling_System.htm
...an expansion volume was necessary in Cooling Systems with no Expansion Tank for this very reason!
Cheers