My strange smell is more like musty or dank, it happened last year at the exact same time of year, we punched out a wall ripped up some carpet and found nothing. After a couple of weeks with the colder weather and less rain it slowly went away. The room in question is an addition and built on a slab.
I don't know what a dead animal smells like, but we have never seen any rats or mice in the area.
Help!

You could have residual mold inside the walls of this room or under the floor due to the rain problem and possibly a leak somewhere in the roofing over this room.

I think you have damp there somewhere.Do you have cavity walls?if so, go around the outside of the building, above the damp coarse and drill holes at regular intervals, take a meter reading, which will tell you wether or not there is any damp present.

If all is ok,it may be that this add on room doesn't get enough sun at this time of year to warm it up sufficiently,try adding a dry air heater or putting a radiator on low setting to keep the chill off.

Do the walls always feel cold even when it's hot outside?Is this room always shaded more or less?if so i think that may be your problem.
In the summer you wouldn't notice it, but when doors and windows are closed and the weather turns damp outside,i think this is when you notice the smell.