Why does the inside of the body freezer at the vet have an odor to it?

If all of the bodies in it are adequately frozen (which they are; that sucker is COLD) there shouldn't be an odor, right? The odor isn't strong or anything and it doesn't necessarily smell like rotting corpses, but I went down to the basement to put the body of a deceased cat into it and a faint smell wafted up towards me. Like I said, it doesn't smell like rotting corpses, but it's not exactly a pleasant odor either.