Saab 9-3 2011 coolant problem

Hey fellow Saab lovers, I have a question as me and my mechanic are completely stumped as to what is wrong with my 2011 9-3 linear Biopower...Last week had my thermostat replaced as it was stuck open, engine light was on...coolant was gushing out the expansion tank and all over the ground, once we replaced it all was good for about a week, then on Wednesday after being in stop-start traffic for an hour we pulled into a McDonald’s and it started boiling out again, not overheating or anything...took it to mechanic and he said the coolant temperature was hot and one of the fans wasn’t turning on...we replaced the fan assembly yesterday and bled the system etc...put a new expansion tank on and it’s still pushing coolant out of the reservoir, we did an emissions test to see if there was exhaust gasses in the system pointing towards a blown head gasket but no exhaust gases were found...water pump seems to be working fine too, we did a pressure test also...We are just completely stumped have no idea where to start looking next