Welcome to the forum its a great place to get your questions answered. Leaks can be hard to pinpoint before settling on the location behind the furnace I would follow the lines and see if the water isn't just running to a low point behind the furnace.
Jake my sidekick (yellow Lab) 10/04 - 05/20
2017 RAM 2500 CC 4X4 Cummins Diesel
2016 Flagstaff 26 FKWS
AF&AM & El Korah Shrine of Idaho