Coolant Leak

Hi guys, hope we're all well I've ran into a slight problem the car has a developed a coolant leak. When in use the car holds coolant absolutely fine, however it's been sat for three weeks roughly and lost just under half a litre approximately (topped it up this morning).
I've had the car in the air and can't see any obvious leaks or any signs of where they have been.
If anyone could point me in any direction of where to start that would be greatly appreciated thank you.