Yep likely condensed water got rotting gunk in. Look for a drainage hole. See if you can fish anything out of it. Bicarbonate of soda is a good mild cleaner/ deodoriser. Maybe look up the manual of your fridge and see what maintenance is recommended.
My fridge smells when I open the door. I've emptied it out and there's nothing rotting away in there and wiped it all down but the smell persists.
Any suggestions on the next step, it appears to be getting worse.