The expansion tank is renowned for getting hairline cracks that are almost invisible, open up when the coolant gets hot and it escapes as steam, so you can't always tell it's there.
Have a good look around that tank and you may see some antifreeze residue.
If its really bad you may see coolant on the undertray, below the expansion tank.
I had the same issue but in fact, when I swapped the tank over it turned out to be the O-ring seal in the base, where it sits onto the lower manifold that had gone hard and was leaking.
Is it auto or manual?
The auto has an oil cooling thermostat in the bottom too. When you lift the expansion tank away the spring inside it can overcome the old brittle plastic and the whole thing whizzes past your ear; many small components in close, but expanding, formation.
So if it's an auto, have a new thermostat to hand too, or like me you're stuck until a new one turns up.
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