In a recent inspection (ocean front condo) I came across heavily darken coils in the air handler indicative of possible tainted drywall but no other indication any place else. All the ground wiring looked good and all the copper tubing looked good. I even pulled out the refrigerator and removed the back plate to look at the copper in the refrigerator and it looked good. I later found out that in previous inspections of other units performed by a company known to do “tainted drywall inspections” in the local area they found the same conditions and in their opinion that was not sufficient evidence of tainted drywall.
My question is: what else could cause the copper coils to turn black in the absence of tainted drywall?
Attach is a picture of the coils. Thank you in advance for your input.