Well, it's the most logical answer. Apparently there were lots of rodents in the walls, making so much noise that my flatmate thought there was a small dog running around upstairs (he later discovered that the upstairs neighbor don't have a dog), and other tenants had been complaining as well, so the landlord announced that he was going to take care of it with rat poison (to save money on hiring a proper exterminator or buying traps). The first few days of the smell, we thought maybe it was some food that found its way under the sofa or something and started rotting. (Actually, I said right away that it smelled like something dead, but my flatmate didn't want to hear it.) We scrubbed all the trash cans. It just kept getting worse. But there were no more rodent sounds. Then my flatmate remembered that the landlord had been talking about putting down rat poison - asked him, and sure enough, he had, but only poison without traps. It's logical that the rats ate the poison, went into their little homes in the walls, and died. We asked the person upstairs, and she said she's had the same smell, but across the hall on the other side of the building the people don't smell anything at all.
We have pinpointed the smell to two places. One is in one corner of the living room (interior wall - it's definitely not coming from anywhere outside), next to the heater. The other is in the boiler room. Logical conclusion: the rats corpses are cooking.
I suggested we just crank up the heat and leave for a day or so, but my flatmate had a fit and said they can't afford that. They only have the heat running about 20 minutes a day, because gas is too expensive, and if we had any money, we'd be staying anywhere but this neighborhood. So most of the time, for now, the room is pretty cold, because I have the windows wide open in there to try to vent the smell. This area is very wet and cold (but rarely below freezing), so I suppose that's just the kind of weather to keep the corpse rotting away slowly for a long time...
If you have another suggestion about what the smell could be, I'm open to considering it, but I can't think of anything else. I have smelled rotting mouse corpse before and I recognize it as that same "odor of death." I suppose, possibly, it could be another animal, but it's most definitely something dead. No doubt about that. Once you know that smell, you don't mistake it for another smell.