DIAGNOSIS: That's condensation and grease accumulation, not a structural failure. The stainless steel bottom pan can develop surface rust, especially if the grill is left covered while still damp or stored in a very humid environment.
FIX: Scrub the area with a brass wire brush (not steel, which can embed particles and worsen rust) or a non-abrasive scrubbing pad. Dry it thoroughly. For ongoing prevention, after each cook, burn off residue on high for 10-15 minutes, then let the grill cool completely with the lid OPEN before putting the cover on.
IF THAT DIDN'T WORK: If the rust is persistent, you can treat it with a light application of a high-heat rated grill paint after cleaning, but this is a band-aid. The pan is replaceable if it gets too far gone.
HEADS UP: This is mostly cosmetic on these models. The critical parts—the burners, cooking grates, and flavorizer bars—are made of more corrosion-resistant materials.