1. See if you can find the cause/source.
Look for flies to help you locate it. Use caution when looking. Other living animals may be around. Also you should wear appropriate protective equipment in case you accidentally find the carcass. Protective clothing should include but not be limited to: 1) rubber or vinyl gloves inside leather gloves, 2) old clothes, 3) properly fitted HEPA filter face mask (consult physician before use).
Sometimes the use of fiber optic probes can help find carcasses in accessible areas. It only requires a drill hole, and if you find the carcass, you can cut a larger hole to remove it. We don’t recommend tearing out the walls because it is easy to be fooled and be one stud away, thus requiring you to make two holes instead of one.
2. Increase air flow.
Air flow dissipates the smell, as does heat as it will speed up the carcass’s decay.
3. Spray deodorant.
It doesn’t have to be anything special. What you like to smell is the most important aspect. Be careful what surfaces you spray the deodorant on as it may discolor the surface.
Remember that the odor will go away eventually. As soon as the carcass dries out, the odor will cease. Of course, the larger the animal, the longer it takes. This need to dry out the carcass is critical. The sooner you can dry it out, the sooner the odor will leave. Of course, if you can remove the carcass (using proper protective equipment and recognizing that there may be infectious agents in the carcass), then the odor will go away even faster.
We have heard of ozone-type deodorizing and/or ionizing devices to dissipate odors. We would advise caution with these devices. The EPA has concerns with the ozone generators. We have also not come across scientific studies on the validity of ionizing devices. Be cautious about such claims from those who sell these machines.
Resources: (online)
http://icwdm.org