You’ve seen it dozens of times… and if you are newer, you will see it dozens of times.
You arrive to your inspection… depending on your order of things, you end up in the garage. You find a Platform for the Furnace/FAU and Water Heater, which has leaked, contaminating the Return Air Compartment.
You pull the air handler cover, remove the filter inspecting the surfaces contained within… you see moldy water damaged surfaces and many times they are still wet.
You can discern what you think it is… but you’re not sure.
Do you operate the HVAC equipment…? As this is a vacant home, you may be contaminating a system with “stuff”. BUT, you have a job to do… the client may be more interested in knowing that their Furnace and AC work, more than your indicating that you don’t recommend running the equipment until the problem is taken care of and cleaned up.
But the question remains… you have a dirty/moldy/rotten platform that houses the return air compartment for the furnace… Do you operate the HVAC?