If you are losing that much but not getting any obvious wet drips from any fitting then my bet would be your rad channels have a pin leak somewhere. Since you have fans on the rad the water will evaporate before it has chance to leak enough to make a drip, if the leak is small. I would also bet that the pin hole is such that with the machine off and therefore no pump pressure, the leak must be so small that it can't naturally drip under gravity either. Traditional way of checking would be to submerge the rad under water and blow into it.