the only reason i can think off is heat and cold exposure... sudden cold then hot then cold again might have cause the dent.. since buyer said no one touched it, plus it is kept in storeroom which sometimes could be hot and humid... plus the day buyer bought it , it was raining(cold exposure).. so prolong humid and hot plus the initial cold treatment.. might have cause the dent...  who to blame... no one.. blame in on humidity..