1994 Nissan Bakkie cooling system

The first suspect is a leaking cylinder head gasket. There's a chemical test that can be performed at the radiator to confirm that, but it's more accurate when the symptoms include running hotter than normal and / or bubbles in the reservoir. Since you don't have those symptoms, you might want to add a small bottle of dark purple dye to the coolant, then check a couple of days later with a black light. The dye will show up as a bright yellow stain that you can follow back to the source. If the head gasket is leaking, you'll find the dye inside the tail pipe.

Auto parts stores will have the dye. Those that borrow or rent tools will usually have the black light too.

Dandy. Happy to hear you have a diagnosis.