JoeJoe, I have seen this before serveral times.
If you are sure it didnt come from the outside, I would say it is plasticiser (DINP, DEHP etc) coming out of the PVC insulation. It's a quality problem from the wire manufacturer that makes it dissolve from the PVC.
It looks like a thin film of oil and can be so much that you see drops hanging from the wires.
The inuslation will eventually harden and lose its breakdown specifications.
You should perform insulation measuring on the site and think about replacing the wiring if values are bad or it looks to messy.
