If all your lines drain, then it's not the septic tank or leach field. If they didn't drain, that would be a totally different issue.
Like mentioned so many times already, you have a leak in your pipes or seals. Since you haven't noticed a leak, that doesn't mean you don't have one. Is the house on a slab or pier and beam? If pier and beam, you need to do an inspection under the house. It's not fun, but it will tell you allot if there's a leak.
Pinpointing the source of the odor will take some time. I'd open every window and close every door. Then go room by room to determine where the smell is the strongest. Some old vent pipes are cast iron and will leak between the seals. There is no fluid in them, just air. If it's a good sized crack, you will smell it in the wall. Fixing it can be a real pain to get to, but it really doesn't matter, you have to do it.
Don't forget the attic. It's not unheard of for a vent pipe to join other vent pipes from the wall and then become just one pipe as it goes through the roof. Where they join could be the cause of the smell. This might also explain it if the smell is all over the house and not just in one spot. Especially if there is little insulation in the attic.
Good luck,
Eddie