Septic tank will likely be 15 feet or less from house. Surely no farther than 20 feet.
Get youself a steel probe. And start poking. Top of septics tank likely 2 feet or less.
Where does your outlet pipe exit.
This first place to look outside.
Septic tanks usually are out rather close. You put loads of distance between outlets pipe and septic tank younafe asking for problems.
Do you know where the field line is?
I am asking this for good reason.
I knew someone who built a garage over their septic tank.. and field line. Septic tank needed pumping and they were SOL.
Had to install whole new system.
Good luck.
They make heavy bars with points to probe around. If ground is not dry not hard to use.