Radiator Fluid going down slowly

Most likely cause is a leaking head gasket. Add a small bottle of dark purple dye to the coolant, drive it for a week, then search with a black light. The dye will show up as a bright yellow stain that you can follow back to the source. If you find it inside the tail pipe, the head gasket must be replaced.

Have you checked the heater core?

Since we know coolant is being lost, add a bottle of dye to see exactly where it's going.

Auto shops should be able to tell you which type to use.

Coolant depleting from recovery tank with no losses from radiator itself is never a head gasket problem.

The most common causes of minutes leaks are hose joints. Check all joints for signs of coolant. When leaks are minute, they sometimes do not leak when engine is hot and at times is the other way round, leaks occurs only when hot.

Auto parts stores have the dye. They will ask what you're using it for so they give you the right stuff. It will say right on the bottle which fluids it can be added to. The bottles are only about an inch in diameter and two inches tall. Many auto parts stores also rent or borrow tools and they will have the black light used for searching for the leak. The dark purple dye will show up as a bright yellow stain under the black light. Follow that stain back to the source. Don't forget to check at the inside of the tail pipe.

Yup, you can pour it in anywhere as long as it will mix with the coolant. I guess I'd wait until the engine had been running and is still warm. The pressure will be gone in 10 to 15 minutes but the reservoir will still be full. Within an hour the coolant will begin to cool down and contract. That will draw some back into the radiator along with the dye. It could take a few warmup cool down cycles but with each one more of the dye will go into the system.

So we are getting somewhere. Good luck.

If it was an external leak from the head gasket it should be higher than the transmission bell housing and easy to see. If this is a V-6 engine there will be core plugs on the back of the block that could be corroded through. That's only going to get worse. You can reduce the leak, as I did on an old muscle car, by leaving the radiator cap loose so pressure doesn't build up in the system. Be careful when doing that because that pressure raises the boiling point of the water in the coolant. It's common for the temperature to get above 212 degrees. If yours does you'll need that cap and the pressure it maintains. That is not a fix; it's a temporary measure to let you get by a little longer before getting it fixed.

Four cylinder engines typically don't have core plugs in the rear so lets rule that out for now. Does the dye show up higher up above where you see the drops forming?

Such problems are due to minor leaks, usueally from hose joints and that is what you should be looking at. The leak is too small to show puddles on the ground.
What you have to look for is signs of coolant which would leave trials oxidised joints, etc.