Something dead in my shed

I am soooo glad we stopped leaving the garage door open for the cat, or whatever it is could be dead in the garage.

I've looked everywhere and can't find it. The smell is overwhelming. I've watched enough CSI that I thought the flies would lead me to it, but there aren't that many in there and they seem to be going nowhere in particular. I wonder if it's a mouse nest and cat killed mama mouse.

Any suggestions?

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