Any small leak will tend to migrate down to the bottom of the block and then along the pan rail and drip at the back, since that is the lowest spot on the engine. You should be able to identify transmission oil versus engine oil by the color.
You need to carefully inspect the engine top to bottom to find any suspect leaks.
If the engine is real dirty, clean it with Gunk and then keep inspecting as you accumulate some miles. Start with the cam cover gasket and work down looking for leaks.