If you have a frost-free freezer, there are a couple of places to check.  On the one I used to have, the bottom of the freezer was a tray that pulled out so you could reach things. The condensation from the "defrost" cycle on the freezer would run to that spot then down the back of the refrigerator wall and end up under the vegetable crispers which were on the bottom.  There was a drain there with a plastic plug where the water drained to the drip pan under the fridge.  Periodically, I would start at the top: pull out the freezer tray and clean under it, wipe down the back wall inside the fridge, pull the crisper drawers out and wipe down the bottom of the fridge, pull the plastic plug and clean out anything there and in that drain, and then clean the drip pan (had to take off a grille piece on the front of the fridge for that, sort of like a toe-kick).  I used a bleach/water solution to kill any moldies that might be growing anywhere.