Do I have a leak?

I opened up a wall up today and exposed the cast iron waste line. At the joint of two sections, it appears discolored both above and below the union. The area was slightly gummy. When misted with water, the water beads up on the dark stain. If I mist the lightly rusted area the water is absorbed. I assume it would be a leak if it was discolored below but above? Is this a leak that needs to be fixed?

The pipes are 40 years old. the section of the pipe above the cement floor has the typical rust for two inches above the floor before the discoloration starts. The cement floor is unstained and appears to never have been wet.

Thanks,

Philip

c. i.

I don't see anything other than the cast iron soil pipe has a rusty surface.
It's cast iron pipe with the tared coating gone and so I think it's just surface rust.

Thanks. So the dark gummy/oily stain was some sort of coating?

Yes it is......