Its probably not leaking at all, its probably just the excess coolant being pushed out which should be caught in the bottle
Get a replacement bottle, original or after market

I've got the same problem. I see traces on coolant on the outside of the overflow bottle. So need to replace the radiator cap and the overflow bottle? its time for a coolant flush anyways.

Adi

[QUOTE=Addy;455734]I've got the same problem. I see traces on coolant on the outside of the overflow bottle. So need to replace the radiator cap and the overflow bottle? its time for a coolant flush anyways.

Adi[/QUOTE]

nah just the cap you replace..
also get your themostat replaced too if your getting a coolant flush

on a GC8 the over flow bottle is not next to the radiator.

it is between the inner guard (skirt) and the mudguard on the passengers side, behind the park light, near the battery.

as already suggested...

replace the radiator cap.

Just get a standard one from Subaru?

Adi

flush your cooling system
undo bottom radiator hose, drain system
inspect thermostat while your at it...down near the bottom hose.
check all hoses.
reattach bottom hose.
put heater onto hot
undo the coolant feed for the turbo from the reservoir and attach a funnel
pour coolant into there until you see the coolant start to rise in the reservoir.
reattach turbo coolant feed to reservoir
run engine until warm and top up as necessary.
shouldnt have to fill up the overflow.
If that doesnt fix it then you have problems.

Yeah just a standard Subi cap.