Hi all i have just replaced my radiator due to it leaking, i have since changed the rad and replaced the coolant and i thought everything was fine.
I went for 2 10 mile journeys earlier today and the water level was holding where it should be.
Tonight i arrived at work and left the car to sit for roughly 4 hrs, i went out to check the coolant level and it has dropped to roughly half level on the expansion tank.
I did bleed the coolant system when i replaced the rad, will leaving the car cause the water level to drop.
The car is not showing signs of head gasket issue's i.e no mayo in coolant or underside of oil filler cap, no excessive white smoke on cold start or correct opperating temp.
The car in question is a 2.0ltr 24v e36 any idea's could there still be air in the system causing level to drop.

If there was still some air in the system that worked its way out during driving, the coolant level would drop once the car cooled off. I wouldn't worry about it unless this apparent loss of coolant continues. Just top up the coolant and keep an eye on the level.

Firstly, the expansion tank is not even supposed to be full to the top. You only fill it to the top after doing repairs and while doing bleeding. Rely more on the floating dipstick or the mark on the tank itself. Half the tank is not really bad. Usually it should be a little above half the tank.
Secondly, most people who do bleeding forget that the heater should be made to work during the process of initial bleeding.\ or immediately after bleeding.
Thirdly, it is normal to run the engine after bleeding and top it off the next morning.