There are tests that can be done to determine if a head gasket failed or not and I assume the dealer did some or all of those tests. I apologize for assuming the dealer did not have their hands on the car based on the way the post was written. The point about service advisors still stands though… 
Subarus have been prone to head gasket problems but if there was no problem when you entered the fast lube and the problem surfaced 10 miles after leaving there it makes things a bit too coincidental.
I have to think that someone did not get all of the air out of the system and allowed it to leave low on coolant.
It could be that the fast lube model of hurry up and get it done did not permit the thermostat to open before handing the car back to you. This could have led to a low coolant level.
The fast lube is likely at fault; proving it could be something else.