Every month my 5lt AU loses about a header tanks worth of coolant (driven few days a week or even all week) and requires top up and weather does not affect this.
I am currently a 3rd year apprentice mechanic and grown up with cars my whole life so im more than confident when it comes to cars though i have no idea where my coolant is going, I have tested everything, its been under pressure up to almost 20 pounds and no leaks, no leaks inside the car, no leaks or cracks or stains anywhere on the hoses, or stains on motor, it has a new radiator as it was starting to go along with the heater tap, before that the upper rad hose is new due to bypass hose to header tank split off, radiator cap is new (Non-Recovery cap, is the right one and rating) along with a new header tank as the old one brittled away bout 3 years ago. I also have a homemade overflow bottle connected to it to see if its releasing out of the cap and nothing.
It runs beautifully, never blows white smoke/steam, and does not have the cooling system pressurise straight away when its ran from cold, i also here timing cover leaks on them, though no sign or stains once again. All i can think of is a CO2 test but almost positive (in the clear) it'll test positive but it keeps going through my head that the motor may be eating it as a result of a blown head gasket/s.
Any ideas anyone?

Header tank cap is too weak.

Yeah but his catch bottle would catch it.
I await your Co2 test.
What's your oil look like?

yeah nothing in catch bottle, bout 3 mils maybe, oils fine, check it often and always clean when changing, however when i did go to pressure test it last time, the tester wouldnt seal properly against the tank, had to use another tester which i only just got to work, maybe its coming out all around the cap not just overflow hose, eith weak cap/faulty tank or a head is the best i can think of

Suspect corroded timing cover if there's coolant in oil...

thats what i'd suspect, had a few in work for that, but oil is fine im sayin, pure gold

Had a similar problem myself for months, for the life of me could not work out where the coolant was going, found a small corrosion hole in the metal heater pipes that pass under the upper intake manifold, just enough to slowly lose coolant but not enough to be obvious...