Fluid leak

Check the transmission cooler lines. They are mostly steel but there is a rubber section crimped onto each one where they go into the passenger side of the radiator. It's by the connections between rubber hose and steel line where they typically leak.

Use a spray-on engine degreaser when the engine is still a little warm, then hose it off after about 15 minutes. When everything in the area is dry and clean, run the engine and watch for signs of fluid seeping from the hoses. There isn't much pressure in those lines when it's in park, so you might have to drive it a few miles, then check it again.