I bought a home with an in law apt that has its own septic in the back of the house. The rest of the house has the plumbing and septic in the front. There is no stink pipe. I don't know how they did it without a stack. Is my only option now is to go through floors and ceilings from the basement and through the roof? Not sure where the smell is coming from. I know its not the kitchen sink. The bathroom is very small and the smell is not overpowering but certainly there. I've stuck my nose against the shower drain and hand sink drain but cant really detect the odor. I will be removing all the floors and walls as I remodel and will have a better idea of how all this is plumbed. Right now it looks like the kitchen sink and bathroom sink drain into the line running from the shower and into the main drain from the toilet and then straight outside to the septic system. Im amazed it is working at all. I'm just trying to figure out how much to budget to fix it. Hoping that I don't have to go inside the house against the outside wall all the way to the roof. The upstairs has already been remodeled
I suspect you have an air admittance valve. Based on your description perhaps it is under the sink or even (improperly) in the wall.
An air admittance valve is commonly used for venting things like islands where alternatives are complex and there is no stack. They often leak sewer gas, though, and are often prohibited for use.
On the bright side if you're going to replumb anyway you should be able to add a more reliable vent fairly easily (though perhaps a little tedious)