Have they tested the coolant? They should be able to sample the coolant to look for combustion products in the water.
Thought my car had a head gasket issue but after the water test showed clear I looked harder, it turned out to be a faulty bleed screw before the heater matrix, after it was at operating temperature you could occasionally see water droplets on the bulkhead where it had squirted out.
My van also looses coolant occasionally, but I’m buggered if I can find the leak. If you’ve been topping up with plain water now is the time to pop down Halfords and get one of those testers with balls in a glass tube, you take a sample and it’ll tell you if your coolant is sufficient to prevent it freezing overnight. Mine is good to -13 as I’ve diluted it by topping up, so I’ll have to top up despite having not found the leak. Should be -37 at full strength.