antifreeze question.

[1tonchevy]

So i did the therma stat an fixed a leaking heater core hose. and filled up my rad as i normally do top it up after completing work,let run with rad cap off and top up as neeed till it "burps" or opens T stat and fill till full. so with it full i put the rad cap on an jobs done. but if i go for a drive or even just let it sit the level in the rad drops not alot but enough to top it up.

The thing is it's not going in the oil, it's not leaking any where that i can see, not dripping on the ground,heater works awsome, and it's not going in the cylenders (i'm assuming as the exhaust is not smoking)

So where is it going? :S

Thanks Dru