You have a leak.
It could be:- leaking into the engine (blown head gasket?), or leaking into the raw water (heat exchanger). Or it could be expanding past the cap seal and dripping out through the overflow pipe.
First things to check are the header tank cap and seal, and the heat exchanger connections and seals. Next check is to top it up and start the engine with the cap off. If you get bubbles coming up through the coolant, you probably have a head gasket failure.
I went through all of this (ended-up having the head off, pressure testing the heat exchanger, the lot). Turned out to be the seal under the header tank cap, which cost £4. So do the easy and chap things first.
Where are you? If you are Solent-based, let me know and I’d be happy to come and have a quick squint at it for you.