Happened frequently to me before I realized that the cap wasn't on tight. I guess a weak cap could do the same. One symptom is...are the hoses collapsed after the system has cooled down? If so, the cap needs to be replaced.
2009 ML350 (84K) - Family vehicle
2001 CLK430 Cabriolet (71K) - Wife's car
2005 BMW 645CI (124K) - My daily driver
2012 Mustang V6 (60K) - Daughter's car