When I was losing coolant on my BMW, it was pretty obvious (and quite depressing).

In my case, it turned out to be a cracked Block (aluminum) due to the car overheating once, in very hot weather.

And a blind man could see the issue, as I was blowing white smoke from the exhaust.

When the block was pulled and inspected, it looked perfect to the untrained eye, but upon close inspection with a magnifier, you could see the telltale hairline crack.

I would think if your losing coolant and suspect an engine tear down, you'd see something in the exhaust, or possibly a wet spot somewhere in the engine indicating a coolant leak.

Perhaps a pressure test on the cooling system will turn up something...???

(Just reread your post ~ you indicate the exhaust is clean ~ disregard the Block...lol...)