We had a terrible odor in our kitchen for 9 days before we figured out it was the fridge. It was an overwhelming sweet urine smell. We were convinced that an animal had somehow gotten into our attic rafters and was urinating inside our walls - it was THAT bad! We went under the house, up into the attic twice and onto the roof looking for evidence, but could find no sign of any animals. I cleaned the garbage disposal, checked all the cupboards, and we pulled out all the appliances including the dishwasher - nothing. I even climbed up on chairs to smell light fixtures to see if anything smelled hot. The odor was so strong at times it made my nose burn. I am very much into cooking/organic, non-GMO foods and we rarely go out to eat, so you can imagine this was quite a serious predicament for us. After several days I was actually in tears over it because we could not pinpoint the source. Finally, after reading some comments on this site, and figuring it had to be coming from something that runs all the time (even though it would come and go, like the cycle of a refrigerator running - duh!), we pulled out the fridge again, waited for it to kick on, and I got down on my hands and knees to smell the back vent. BINGO!
A month ago we'd had strong straight line winds come thru and we were without power for 48 hours. We quickly unloaded our fridge/freezer and moved our food to a relatives house. However, a couple of times I opened the freezer door and there was a brown liquid running from the back left corner (under a small enclosed compartment of some kind at the upper left) to the front that I wiped up with paper towels. We have no idea what it was since the freezer was empty, but it happened 2-3 times and I just wiped it up without giving it another thought. Apparently, whatever this liquid was, and wherever it came from, it must have drained down into the drip pan. When our power returned, I had already cleaned the entire fridge before putting everything back, so all was spic n' span. Two weeks later we caught the first inkling of a strange odor which just kept increasing as the days went on. We called pest control, who said they couldn't help us, and were just about to phone a wildlife expert to come to our house, which would have cost us hundreds of dollars, when we finally discovered it was coming from the fridge vent.
Our Frigidaire is still under warranty and the repairman came today. For some unknown reason, refrigerators are made with non-removable drip pans now. We had to tilt the fridge sideways, my husband held it at an angle on one side while the guy took out two screws so the pan would slide out. It had this putrid brownish liquid with white blobs of goodness knows what floating in it - sickening! I got rid of all the crud, washed and dried it and he screwed it back into place. Problem solved - we are very grateful and relieved to have the mystery and stench behind us!
If you figure out that this is your problem and don't want to spend the $$ for a repair, take everything out of your fridge, get someone to help you tilt it, remove the two screws and take care of it yourself. If your fridge is older and the drip pan is removable, just follow your instructions booklet for cleaning - it's very simple and easy. Hope this helps.