You can't find the leak their is no puddle?

your heater core has an internal issue. if that is not it, then its leaking into your oil, an easy way to check that is drain a small glass cup full of oil. now oil floats on top of radiator fluid so if you have any discolored part of fluild in the cup then you have a blown seal in the engine. but this is a "just in case" type of thing, i doubt it happened to you. but the easiest way to get this thing solved is run a diagnostics on it, autozone is free, and even though nothing shows up on the dash, the computer might know something you dont