Coolant Leak

Folks my Octavia (1.6fsi) has developed a slow leak in the coolant system: over about two weeks the reservoir drops from max to min. There's no sign of water staining around the engine bay and the oil is not milky or gunked up (and the car gets up to normal operating temp fine and the heater works properly).

I'm just about to book it in to have it checked over, but would anyone have ideas as to a possible cause (as these sorts of problems can be a awkward to diagnose and any help would, perhaps, expedite a solution)?

Cheers.