Mate i've had the same issue which drived me Nuts.

Workshop did pressure tests and everything and the mofos couldn't find it but happy to charged me everytime. At the end, i undo the plastic cover under the engine bay and found coolant was leaking where the Transmission Cooler is.

Valley Pan is that last place you'd want to see the coolant leak as i did some research and found its hard to do ..... offcourse that's just me.

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13.6 Street tire Gtst
11.6 Street tire R32 GTR