Since the coil was above the furnace, it is conceivable that condensation is the source of moisture intrusion into the lower cabinet and burner assembly. The drain may have been clogged and overflowed, or the lower section of the evaporator coil assembly has rusted through.
Many times you see three simultaneous conditions: a rusted evaporator coil assembly, a partially or completely clogged drain, and a cracked heat exchanger. All more or less age related.