I seems to me that smoke would have no place to go except up a vent since the septic & cleanouts are buried.
A metal detector might find a cleanout if there is any cast iron.
The building department should have a record on file from when the house was permitted
5 years is probably too long between pump outs
You can bet the septic cover isnt air tight.It's probably covered with dirt. The smoke will find the cracks around the cover.
Cut it 3 times & it's still to short.
Inventor of the Miter Master Plus.