Taking a "morning" look at your post, I am guessing you have a hairline crack in the plastic part of your radiator, on the top, right side. The reason you see only a little bit of coolant is that it only leaks when the engine is hot; and since the hairline crack is at the top. You can check for this two ways (1) check for wetness at the top of the radiator immediately after a long trip (2) look for wetness under the hood near the front on the frame, block etc. The wetness may look oil like, as the coolant slowly combines with the oil and dirt on the frame and engine and looks like thin oil.
We need to know your mileage and if you have ever changed your LIM gasket. You will be replacing the LIM gasket as your 4.3L engine approaches 70k plus miles. Finally, the water pumps also fail on the 4.3L, a new OEM pump runs about $40.
Assuming the LIM gasket is original and the radiator is the source of the leak, I would consider replacing the LIM gasket, radiator and water pump both as a repair and as a preventative measure. Total parts cost (Delphi radiator $100, FELPRO LIM Gasket set $60, AC DELCO water pump $40. All parts from rockauto.com . $200 in parts (plus your labor) and coolant.
All the above parts are interconnected. Changing them out at the same time will actually save you time and money in the long run.
2001 Bonneville SSEi, 219,000 miles.
Whats in my driveway:
1998 Chevy S10 4x4 ZR2, 1999 Olds Bravada, 2001 Pontiac Bonneville SSEi, 2007 Ford F-350 regular cab 2WD, 2008 BMW 335i coupe (Wife'* replacement for her Bonneville). The Bonneville is my favorite of all the cars in the driveway.