[PeterW]

It is very normal to top up between service (once a year), but not more than 1-2 liters.

I have experienced it was completely out of oil (think they had forgotten to fill up after service) but did not get a warning, discovered it myself when I checked the oil.

 

[PeterW]

This car only contains 3.3 ltr oil, therefore it´s extremely important to fill it up regularly.
If it is only 1/2 filled then you drive around with under 2ltr oil which is not enough, you will also experience it running better when the oil is filled up.
Citroen changed my engine as it used too much oil, up to 0.5-1LTR per. 1000km.