Can bugs actually live inside a freezer? Found a mess behind the back panel

Those are almost certainly pantry moths, probably Indian meal moths based on the size and the little oval cocoon bits you're describing. They got in through food somewhere in your kitchen, not through the fridge itself. The drain tube and drip pan theory makes sense for how they traveled, since that channel stays above freezing and has moisture, but the original source is almost always a bag of grain, flour, pet food, or birdseed sitting in a cupboard somewhere.

First post here so hopefully I'm doing this right. We had pantry moths last year and ours came out of a bag of hamster food in the hallway closet, nowhere near the kitchen. Took me ages to figure it out because I kept checking the cupboards only. One thing I'd say is check anything cardboard in your pantry, they chew into boxes and the eggs hide in the seams. I also froze everything I kept for a couple weeks and that seemed to stop them hatching. Does anyone know if the cold actually kills the eggs or just makes them dormant?

Agree with the poster who said the source is food in the kitchen. We had the same thing behind our freezer panel a few years back and I just vacuumed everything out with the crevice tool and wiped the fins down with warm soapy water on a rag, let it all dry good before putting the panel back. That fridge ran fine for years after. Only thing I'd add is put a couple pheromone traps on the counter, if they still catch moths after a few weeks you know there's still a food bag somewhere feeding them.

For the drain tube part, a turkey baster turned out to be the thing that saved us when we had the same mess. I mixed up hot water with a splash of vinegar and flushed it down the drain opening a few times untill the water came out clean in the drip pan underneath. Pulled the pan out after and washed it in the sink. Never would have gotten that channel cleared with a rag alone.

Update: finally got a free evening to deal with this and it went easier than I expected. Vacuumed out all the gunk behind the panel, wiped the fins down with a damp rag, and the turkey baster trick worked great for flushing the drain line until it ran clear out the bottom. Also did a full pantry sweep and found the culprits in a bag of bulk oats I forgot I had. Tossed it and no moths since.