I saw brown marks on some Romex® in my basement. It was also on some spots where it touched insulation. I nearly freaked out until I Googled and saw similar stuff like this or this and people said it could be just warm enough to pull the tar from the insulation or could be sap from running through studs or even condensation. It was easy to access so I replaced it and spliced it open to confirm it was on the outside of the jacket but not the inside. Have any of you seen stuff like this?