I have had a hairline crack in the same spot on 2 GM radiators. It was in the top plastic section on the drivers side. Coolant would seep out only when radiator was full and hot. It would come out slowly, mix with the dirt and engine grime, and look just like engine oil at the bottom of the engine and frame. These cracks were not on my Bonny, but on a Bravada and a S10, but my lesson learned was that slow leaking coolant, when hot and from a high location, ends up at the bottom of the engine and frame and looks just like motor oil.
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.