Odor around air compressor

Im sure you dont have access to an LEL meter but do you have an AC refrigerant sniffer? Maybe sniff around the area and see what you find. I know my refrigerant sniffer will pick up gasoline and probably anything with hydrocarbons.
Rusty metal can have a funky smell to it. At only 6 years old and the fact that you drain the tank you should not have any significant rust issues.
Humidity sucks! The last couple days have been brutal here...first taste of summer.