Head Gasket problem?

I just got an engine swap on my 99 civic ex and got a JDM GSR engine. I have been having coolant loss for a while now, at first it was leaking from the radiator hose so I got that tightened up. However, the coolant is still consistently going down. The engine never overheats though, from idle to WOT the engine is always steady on normal operating temperature. I know the coolant in the plastic goes up and down depending on the temperature of the engine, but just 2 days ago I filled up the container to max and now it's way below minimum. Where is this coolant going? I check the radiator every morning before I start it and it's always full, I dont see any leaks, nor bubbles in my oil, nor green tint on my spark plugs and no white smoke from the muffler. I dont' smell any sweet smell or antifreeze odor either. Although there is a a sharp whistling sound when I hit the gas. And I dont' really notice any power loss either.

I got the engine compression tested and leakdown and was top shape when first got it, it still runs great but losing coolant.

Should I be worried about a blown head gasket? What steps could I do myself to determine the problem? Compression test and what else? I was thinking that maybe my stock radiator (which is almost 7 years old - civic ex) is not cutting it with the integra B18c in there. Help please!