try powdered lime

I'm wondering if you didn't seal the wall again prematurely, although I couldn't say if another 'critter', or if you just closed in the odor so its taking longer to fade...

My former house had an unfinished half basement used for storage. A rat was able to get in to the insulation batting in the basement ceiling from the crawlspace under half the house that was dirt. Several times apparently, there were multiple packets of those envelopes of rat poison that someone (we never did find out which neighbor) had been tossing around for rodent control.

Directly below our bedroom closet and just a few feet from the head of our bed. DH fled to his fathers house (lots of help) and the exterminator from the pest control company and I found the rat body after much hunting and testing. We pulled down that insulation and more debris, torn open poison packets, along with the dead body fell down on us than I want to describe.

The odor dissipated overnight once the offending carcass was removed and with the help of a fan. Any conceivable space allowing another entry - carefully permanently sealed.
The exterminator had an odor-covering spray that we chose not to use and turned out not to need.

If the rat decomposed it's possible fluid seeped into the surface upon which it lay. That material would have to be R & R'd. One of my relatives died and decomposed onto and into the hardwood floor. Floor had to be replaced. Luckily the floor joists did not.