Coolant level low - top up enough or need to get it checked for leaks?

My coolant level is slightly low. The on-board computer notified me of this about 3 days ago. The warning light comes on only intermittently i.e. not every time I start the engine/drive the car (2006 Vauxhall Vectra Elite, 2.2L Auto petrol).

Is it sufficient for me to merely top up the coolant/antifreeze, or do I need to also take it to a garage to get the system checked for a leak?