MG ZR reassurance needed, possible hgf

I've owned my mk2 zr160 for nearly 2 years, 100k on the clock and no history of the head gasket being changed. For the last year or so I've been topping up the water about 0.5 litre every 3 to 4 weeks, so a very small loss and no obvious sign of where it's leaking from.

There's no sign of froth on the dipstick, the oil filler cap shows a very small amount but I've just put that down to condensation from the short daily drives I do, and the expansion tank shows the coolant slightly cloudy but nothing obvious.

The head gasket has come into question since we put a pressure tester on with the hope of spotting a leak but we couldn't see anything, not even inside the car from the heater pipes, yet the system definitely lost pressure. So now I'm beginning to think it's hgf, but before the head comes off, is there anything else that could cause the water loss that wouldn't show a puddle or obvious leak?

Thanks in advance

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