Septic tank might be under an addition

[Brian Barfoot]

So I have a property that's getting its garage converted to a studio. Part of this is to add plumbing and so the plan was to tap into the existing septic line. The trouble is we can't find it. This house is an amalgamation of additions (from before me) and I worry that the back section of the house was built right on top of the tank. 

Is this something anyone else has seen? Is there some kind of tracing tool I don't know about that can help us figure out where the line goes? I half considered putting a gps tag on a fish tape and sending it down the cleanout but those things don't have great accuracy and I would want to know that path to more precision than six feet. 

[Bruce Woodruff]

This happens a lot...like @Bjorn Ahlblad said, you can just go to your County Dept and ask for the original plot map. OR if that fails, hire a plumber that does sewer cameras or pipe locating, they will have devices that they can track...