Bleeding air out of cooling system?necessary?

[omodos]

Hi, have an issue with my x5 3.0d got no visible coolant leaks yet say after about every 100km, when I check coolant level the bob has gone from max to min, I top it up with very little cololant and it reaches max, so the loss is not huge. There are no visible signs of leaks, yesterday I topped up again but this time I read the guide on the bmw coolant bottle, it said turn the heater on and observe the effects of the coolant?anyhow I turned on the engine n heater, and added the miniscule amount. The heater I hardly ever use cos it is so hot out here, so could there be an air lock in the system that I simply cannot get rid of giving me the low coolant level?and if so how do you get rid of air locks, is there a vent valve?or do I top up, run engine for a little while then let it cool and remove expansion tank cap to remove air?any tips would be m,uch appreciated