Radiator leaking when car is not using for days. if on engine no leaks

please help me what is the possible solution for this?

the circle on picture (near the petcock) has water leak when the car is not using for days. the water from the reservoir tank is draining.

but if i use the car. it doesnt loose any water. even long drive.

salamat ng marami!!

thank you!

the car is honda civic ESI 92-95 model75534884_950544615311119_6672065607950663680_n.jpg