Dead animal in the wall?

My contractor fixed the patio door the other day. The casing was rusted. Yesterday when I went to the house I smelled a very bad smell from the wall from inside the house. We have a lot of squirrels, chick monks etc around the house. I am thinking that one or more of them must have got in the wall from the opening. When the door was fixed, it must be trapped in there. But this is just a guess. Has anyone had similar experience? Do you think it is a dead animal? The contract took out the casing and did not see anything from outside. Should I open the wall from inside? We do not know the exact location. Will have to guess and probably make a big mess. Thank you very much!