Literally just posted this in another coolant leak related thread;
I recently had a very slow coolant leak happening. After 20 miles or so, no noticeable drop in header tank, but after a good run (few hundred miles for example) the level would drop to about halfway between the low and high.
Originally thought it was the "coolant plastic flange" on the right hand side of the engine block and/or the thermostat housing. Replaced these earlier in the year and thought I'd fixed it. I hadn't!

Did a bit of research and the other classic leak location is the "plastic y-piece" on the left hand side of the engine block, next to the charcoal canister and kind of underneath the header tank but slightly forward. I paid £30 for an ECS tuning metal replacement from PSI tuning and fitted it at the weekend. Still need to do enough miles to tell if its actually fixed but the give-away was some pink crust building up around where the hoses joined the y-piece.
HTH?