Dead mice/ rats are no fun!
On my last farm, we were plagued with mice & rats due to all the cattle feed.
The (ex)wife really loved the D-con, much to my chagrin!
Had a few mice die, one in a wall in the Living Room. Couldn't get to it, but I narrowed down where it was in the wall, bored a 2" hole in the wall & dumped a LOT of Hydrated Lime in the hole, patched & repainted, then caulked the entire length of that wall along the floor. That helped a lot.
Here's where we enter a NN-type story:
Noticed an odor of dead rodent in the same LR a few years later, stronger & stronger every day. The odor finally became unbearable.
Of course I had moved all the furniture, checked behind it, inside it, even removed the lining on the bottoms to check. No rodent found.
Finally decided to go upstairs & check the crawlspace, BINGO!
A large rat had died almost on top of one of the can lights, so far back that it was unreachable from the crawlspace.
From the LR, I was able to remove the can light, (wearing a full respirator, latex gloves covered with leather gloves, & long sleeves taped to the gloves!) reach up through the opening & drag the remains out.
Man do those things rot fast! It already had maggots in it!
Scraped, wiped, bleached for what seemed like hours. When I got all of it out that I could, threw several handfuls of Lime as best as I could on & all around it between the joists.
Took a couple weeks with an exhaust fan in the window & lots of Febreeze to fully get the odor out!
Needless to say, D-con was then forever banned in my house!
Spent the rest of my time in that house baiting & setting Mouse & Rat traps, checking them frequently.
Now I'm on a different farm, (with a different wife,) same Mouse/Rat problem, we set mouse traps in the house (Have no idea how they get in!), but we also got 2 cats in the barn.
It's been forever since I have caught a mouse in the house, and I see maybe one a week in the barn.
NN, may I suggest placing a pan full of Febreeze inside your return register?
Just get the Febreeze, dump it into a pan sized to fit your duct so as to hasten evaporation, (be sure to set your fan to "run" rather than "automatic" on your thermostat).
Or, just spray your filter(s) as often as possible for as long as it takes if you can't get into the actual duct.