It was explained to me that the bottle was an overflow tank. There is no pressure in it because it’s not sealed, it has a push top lid but that lid is just pop off pop on tight not screw on. I always fill from the top of the engine, like you would on a car, unscrew the cap and poor the coolant in. Could it be that there is air circulating that once finds its way out, it sucks some more coolant in? I’ve noticed a drop in level ever since I re fitted the turbo.