Funnily enough I noticed mine had some water in it last week and had to soak it away with paper towels as there is no drain plug,it wasn't smelly though.
it normally just evaporates away due to the proximity to the hot compressor at the rear of the fridge - you get problems though if something leaks or spills in the bottom of the fridge (e.g. milk or meat juices), which then run to the drip tray and the warm environment allows the bacteria to grow.
Fishy smell? Could be an electrical issue heating up some plastic somewhere.
We have a CO detector nearby, would've thought would go off. But certainly not a gas smell. Not sure what damp/mildew smells like lol