Hello I will help you with your question,
These cars are prone to head leaks but your may be leaking correctly. See if the bottom rear of the right side head is seeping slightly and dripping on the exhaust where the coolant is flashing to steam. This may be what you are smelling.
Subaru dictates the use of "cooling system conditioner" (leak sealer) everytime the cooling system is drained so if this was not replaced you should install it to the radiator and run it awhile to see if this seals the system.
If you are having to "top off" the cooling system you do have a leak but if all you have is the smell then it may just be seepage and the conditioner will stop this. If the smell is from the exhaust run the spark plugs for 1000 miles and pull them. if they are dead white they are being steam cleaned and the coolant is being burned so your next step would be the gaskets.
Let me know how I can help you with this