Yeah, in terms of the squirrel thing... If we assume that the squirrel explanation is 100% false, then we can ask why she picked that as an explanation for the smell. I think my point ended up being somewhat convoluted, but what I was trying to say can be expressed in this fictional account of Casey's internal monologue.
Casey's head:
"Well crap, Lee is asking why the car smells. And it stinks because there was a dead body in it. But I'm not gonna tell him that, ha. Ok, I'll tell him something that seems believable. Ok, what's stinky like a 2.6 days of dead-body-stank but doesn't put me in jail? Hmm, how about rotting engine-heated squirrels? I bet that's similar. Yeah, that's the ticket."
So my proposition is that you would only make up something like the squirrel story if you were trying to cover up the fact that it was something else rotting and of animal origin. Thus, clearly she was trying to cover up that it was something else rotting and of animal origin.
Or, maybe it's just that trash or rotting foliage or food or skunk wouldn't have been remotely believable. And, actually, maybe one could make the case that if she were trying to cover up that she had a body in there, she
wouldn't have said anything remotely similar to a dead body, because that would be too obvious, and she's not that dumb.
Or, maybe I'm over thinking this.