Where is the poo?

Classic situation. People buy a house with a septic tank; ask the idiot previous owner where the septic is located; idiot does not remember; people don't press on about it, because they have buying a house on their minds. A year later the loo does not flush really well, or there suddenly is water in the bath when the washng machine does it's rinsing cycle. And that water is not exactly fresh, I can tell you. You either have a blockage somewhere, or your septic is full and needs emptying. But, where is it?

We have two septics. One we know, it has a hatch that is visible, the other is behind the house, but we don't know where exactly. There is a pipe leaving the house at the red arrow, disappearing underground. The other, greenish here, enters tha garden wall at the green arrow. We dug some holes, but to no avail.

Then suddenly, while walking the dogs, I thought of the plant container at the edge of the terrain. We don't have many plant containers, and this one is about septic tank hatch size. There just is quite a large palm in the middle of it, with pebbles around its roots, as you do in a plant container.

Decided to dig next to it, to see, and yes, it does extend underground. Let's see how we can remove this palm..