From what I've read, there's a wealth of collective knowledge here.
I own a brick 2-story home built ~ 40s... Periodically, we have an awful smell of sewage coming from our basement. This IS periodic, and seems to come and go throughout the day. I can't seem to link the smell with anything we're doing in the home, but when we try to remember back to last winter (we JUST bought the house), we don't recall this being much of a problem.
The basement is unfinished and has three rooms. Each room has a floor drain - the room closest to the street (where the floor drains go to) has two floor drains. The house has one pipe that comes from the 2nd floor utilities and one comes from the 1st floor - each travel through the floor of the basement. I'm not very plumber-savvy, but I'd have to guess that the floor drains are not connected to these.
When the odor is detected, we rush to the floor drains to try and find the culprit, but I can't seem to find it! We typically pour hot bleachy water down the basement floor drains, but this is mostly because I have no other ideas. The bleach, if anything, gives a more pleasant odor to the basement. Other than this, I can't be certain this is actually doing anything. The floor drains have water poured into them at least once a week, so I don't think they are drying out.
Basement very rarely gets water in it - we have no sump, but we do have a basic interior basement perimeter drain (which oddly enough, does not drain anywhere). We are in the city, and do not have a septic. Not sure if this information helps...
We've recently had many of our house pipes snaked out - including from the basement floor drain to the street. No change.