Not sure if this will be the case,but we had a KA that lost water, but no obvious leaks or head failure.

Turned out to be a common issues with the cap on the expansion tank, the rubber seal in it perishes or gets twisted and doesnt create a seal and so the water evaporated.

I refitted the seal flat again and it solved it. I did find out though that the fan doesnt kick in until the water reaches 105 degrees, which seemed odd, but when I let it run longer, it did kick in.

Was shocked out how much water evaporated and how such a small cheap fix sorted it.
Worth a check.