My septic tank is backing up and no one can find it! One plumbing company spent 4 hours looking and finally gave up, they even snaked the pipe on the roof! The previous owner says he removed a large tree where the patio slab is now located before pouring the slab and thinks it may be under the slab. I always thought you wouldn't have planted a tree over a septic tank to begin with. My neighbor says that her septic tank is out about 10' from her house to the left. Help!!

Since poured slab, little tough digging around. If it was me, (I am Electrical/Electronic)
I would push fish tape through roof vent, and hook up signal generator and locator.
Progressive Electronics makes a pair for about $200, couple of years ago.
Maybe the county has a record of it?