Today, many heating furnaces and air conditioning evaporator coils are installed in the attic.
It frees up closet/storage space in the house and it’s quieter when the heating/cooling is on.
When an air conditioning evaporator coil unit is installed in the attic (the silver box in photo), a condensate overflow pan is typically installed under the unit. The condensate overflow pan is a safety device intended to prevent unwanted condensate leakage (water) from spilling onto floors and ceilings, if something goes wrong.
In the photo taken during a Walnut Creek home inspection, there's rust colored water in the overflow containment pan and the pan appears to be corroding... not a good sign.
If left unchecked, the pan will eventually rust through. This can lead to mold growth in the attic insulation or drywall ceiling; and/or ceiling water stains or leakage onto a floor below.
You can count on the The House Whisperer to see red (in the water containment pan).
I recommend further review by a licensed HVAC contractor.