May be an "off the wall" suggestion, but it happened to me . . .

I had symptoms similar to yours once, and I just could not find a leak anywhere. Yet, I would continue to find "old" stains of antifreeze on the engine block near the thermostat. Finally, I found a very small tear under the clamp on the top radiator hose. With the engine off, it did not leak, but running, under pressure, it would blow just enough antifreeze onto the block to keep me confused. You might check it out; who knows.