Normally on the side of a newer furnace install, the installers will have the maintenance, warranty, user manual, information fastened to the side of the unit in a handy little packet. This way there always there when you need them. Now on this particular furnace there was some very important heating & cooling information in this packet. Can you see it?
Yup, that's some dirty water. But where it did it come from? Well, first understand, this 1900's house didn't have heat ducts in the upstairs bedrooms, just a hole in the floor for gravity heat. So when the new furnace was installed, the HVAC technician decided to upgrade and run heating & cooling ducts to the upstairs. The following pic shows how the installers ran the heating ducts to the upper story.
Straight up the roof and into the upstairs attic. Yes there wasn't any kind of flashing installed just cut a hole, run your duct, and seal with the universal fix-all, caulk! Caulk fixes everything, don't you know?
Sure enough water had found it's way in and around this exterior plenum and infiltrated the structure below. The evidence happened to find its way into the information packet on the side of this particular unit. A sign from the heavens? I think so.