engine SOHC and Radiator question

i have an SOHC engine and recently i have replaced my OEM radiator with a HASTE 3 row aluminum radiator..
my question is when i travel like in long distance the water in my reservoir tank reduces..like half a glass (more or less) i know the radiator gets water from the reservoir or drains water to the reservoir but it is more likely it gets water from the reservoir.. i guess the real question is where does the water go??
i always check under my car for leaking from the radiator and reservoir and no leaks..
i also always check my dip stick and oil cap for water mixture and thank God there is none..

any ideas whats happening?
pls do share some thoughts..
thanks

The level in the tank should go up and down a bit due to expansion.

It should not need topping up though.
I expect you have had some air in the system that's working its way out causing the level to keep dropping

Did you re-fill the rad with heating on? If not turn the heating on next time you run the car.

Is there a bleeding system for air in the radiator system or somewhere near the thermostat?

Im sorry but im not quite familiar with the heating on...are you refering to the Aircondition heating setting?

The heating in the car is "powered" by the engine - when you turn the heating on a valve opens that diverts some of the coolant from the engine to the heater matrix in the cabin where it heats up the air pumped by the blower motor - thus heating up the cabin. When you changed your rad some of the coolant would have escaped from the heater part of the cooling system and if you don't turn the heating on when you re-fill it it can't bleed properly - because the valve is closed. So turn the heating on (no need for the AC to be running) and run the car for a bit, this should hopefully bleed the system. That's what I always do and I've never had a problem.
Another possible reason could be a minor leak somewhere, check the fittings on the rad. If the leak is very small you may not see any coolant escaping as it'd evaporate straight away.