What you described is precisely an issue that I recently had though not in a Subaru. And the coolant stayed clean. It was a one way leak of exhaust into the water jacket. I could pop the coolant overflow anytime after driving and find bubbles coming up in there. I’m not precisely sure that this was the dynamic, but basically the coolant would expand and hit the overflow as normal. On my car, as the coolant cools & the level drops in the radiator its kept full by siphon back out of the overflow. The exhaust bubbles would rise to the radiator cap & kill the siphon. So I was never really losing any coolant, but the radiator was getting low b/c it wasn’t pulling back from the overflow.
You should be able to verify this easily - after you drive it next time, just leave it running when you stop, pop the hood an take a gander into the overflow for bubbles. If you don’t see any, then start looking for an alternative explanation.