I would also have thought 2L isn't a worry coolant concentration wise,unless i was in Tasmania this time of the year,where there is a chance of freezing the coolant.
Grab a torch and small mirror,and have a good look over the engine with the cover off,first thing in the morning when it is cold,for any weeping or signs of s coolant leak.
I doubt there is an issue with the block or head,well there could be but you would have to be very,very unlucky.
More then likely if there is a leak,it is a gasket,hose or fitting.
Some also have some plastic parts of the cooling system that can also be an issue.

