Rust on the bottom of my Furnace.

Master

I'm finding major amounts of rust on the inside bottom of my furnace. I thought it may be from the air conditioner condensate pan below the 'A' coil. I opened that up but no evidence of leaking appeared. Now I am seeing a small container that will hold water located in the furnace next to the main blower motor and plumed with pvc to the drain, this may be where its leaking. Hope someone can help. What is this thing ? it says... Caution fill with water each season bla, bla, bla. Do I need to do this? What is this thing ? oh the furnace works fine. Thanks Mike.. Master Electricain. The furnace was new in 1999 and is made by Comfortmaker.