First, separate “not found online” from “never permitted.” The file may use a former owner, subdivision identifier, rural route, or old address. Ask TDEC to check the clues you have. Tennessee began digitizing historical septic records county by county around 2017, but a rolling scanning program does not promise that every older installation has a complete searchable layout.
There is no defensible statewide rule that every pre-1970 system lacks a record. Treat a pre-1970s house as a higher-risk search because its system may predate modern documentation, may have been replaced, or may use an old location description. The year is a clue, not a conclusion.
If the file remains missing, hire a qualified septic inspector, pumper, or installer to locate the tank and field on the ground: trace the building sewer, probe carefully, open the tank safely, or use locating equipment. Keep heavy equipment out of the suspected field. Never enter a tank, and do not dig blindly near electric, gas, water, or other buried utilities.