For 500ml in one week ? Radiator core 90% of the time if you can't see any obvious leaks.
I assume that you have had a sniff test done via the coolant expansion tank just to eliminate the possibility of head gasket, although this only eliminates one part... The mixing of oil/water channels and doesn't factor in escape of coolant via the outer gasket seal to the block itself.
Remove the front bumper, not as hard as you would think.... also remove the fan cowlings to the radiator... Give the radiator a good inspection. You are looking for damaged fins with a tell tale fluffy green residue... If you find any, then there is your culprit.
Don't be fooled by the fact that you can't find a physical dripping leak... When the car is up to temperature and air flow is coming through the radiator, the temperature of the core actually gets hotter than the coolant. So as it escapes through a corroded area of the radiator core, it dissipates in a small steady stream of steam a little at a time... So much so, you wont even notice it , especially on a motorway.
Equate the 500ml to a weeks worth of driving and say you do 500 miles in that time for sake of argument, then you are loosing 10ml's per 10 miles which on the face of it, isn't a lot at all.. So no wander you can't see it or find it.
Imagine if the coolant pressure is 10psi in the system... And also bear in mind that the smallest hypodermic needle on the end of a syringe can deliver 10 ml in a matter of seconds when at the doctors, you now have an idea of just how small a hole needs to be to loose coolant at the rate you are 10ml @10miles= at say, 60mph is 10ml every 10 minutes!
It is always, always good practice to replace the water pump along with the cam belt etc if it is due anyway, so don't think you are loosing out by changing it... Just be mindful that while they have the coolant system drained, its also far more cost effective to at least get the radiator inspected or changed along with a thermostat before they refill the system with fresh coolant
A radiator has an average life of 5 to 6 years before core failure assuming nothing has damaged the fins before hand
Hope this helps