If it truly is a "dead" smell, it could very well be something did crawl up into your car and die. Since you are smelling it over and over again, I'd suggest it wasn't something small like a mouse.
The only way to tell for sure is to pull your car apart. This is going to be a bit of work, but if you want to find the smell, it's about the only thing I can think of. Since the heater being on eliminates the smell, I would put money on it being on the inside of your car and not inside your HVAC system. Usually when that happens, whether mildew or what have you, the smell will get worse with HVAC usage.
From my point of view, you'll need to take all of your seats out, one-by-one. Then inspect the seats to ensure nothing is in/on/under them. Once you pull the seats out, leave them out and see if the smell comes back. What you are doing in this process is eliminating where the source of the smell might be. Luckily, most seats are fairly easy to pull out. The front seats usually have four bolts/nuts which need to come loose, then an electrical attachment (disconnect the plug). The back seats may be a little more difficult, but I'll bet this is where you'll find the smell emanating from. There are different ways back seats are held in, but usually you can pull the bottoms from the back (by the seat back) and they will come out. It's more of an interference fit.
You said you don't drive the car for several days, which implies to me this is not your only source of transportation. It's the only reason I'm suggesting you go this route. You could take it to someone (a body shop) and have them do the extraction. They will easily know how to pull the pieces apart quickly and without damaging anything. They'll also know how to get it back together properly. I could not even begin to tell you how much this might cost, but I'm sure it wouldn't be cheap. Expect to pay for a full days (eight hours) worth of labor at the going rate.
This is the best I have for you.