Photos that show the layout of the slab relative to the house and property could shed more light.

I say we excavate along side the car port, then tunnel horizontally under the concrete until we find the “treasure”.

The septic tank would need to made of steel to support 4 ~ 6" of concrete.

I've seen septic tanks with one and two access points. Round covers, square covers or one big tapered rectangle "plug". Cement tanks, fiberglass ones and steel. Square, rectangle and round.

And we're off topic.