In the absence of any external leaks (hoses, water pump), the most obvious and common diagnosis would be early CHG failure. At this very early stage of failure a pressure test will be useless (a vessel with a tiny puncture can appear continent and hold pressure for several days). The garage should have tested for the presence of exhaust gases in the coolant, but if there is a 'one-way-valve' effect even this test might be negative.

It is always difficult to know what to do, as the tiny leak might not get much worse over the next few thousand miles, or it might deteriorate rapidly. Addition of sealing substances e.g. 'radweld' is debatable. Running it unpressurised (coolant cap loose) will reduce coolant loss but risk blowing it out if it gets too hot. Unscrupulous owners might sell at this stage, well topped up and denying any problem.

Gasket leaks can have different symptoms, depending which galleries have become connected. Is there any 'mayonnaise' on the oil-filler cap ?

Several years ago I suspected a small gasket leak, and my trusted indy did a pressure test with negative results. In the end I decided to change the gasket which cured the problem.

i would have a real good look for leaks .. get underneath. strong torch. mirrors on sticks. tissues . with and without engine running . at temperature and cold.. ,. only going to get worse and let you down

As mentioned above, if it's a 1.4 petrol engine, I wouldn't rule out external leakage. Our previous Corsa had a slow but constant coolant loss at a rate similar to yours.

I ruled out HG failure as the coolant was still bright pink with no oily spots, and there was never any mayonnaise under the filler cap. Eventually I spotted a drop of coolant on the sump on the water pump side of the engine. Local indie checked and found a small leak from the water pump housing. He replaced the water pump, problem solved. I seem to remember it's a fairly common issue on the GM 1.2/1.4 four cylinder motor.

The leak was small enough that the coolant was being evaporated on the hot engine, which meant it wasn't dripping on our drive. I only spotted it when I did because the car had done a very short run, and the engine hadn't got hot enough to vaporise the drips.