The lids to septic tanks are often buried underground. Where that's the case, it would not be possible to fall into a tank, and it would be obvious from the dug ground if it had been recently accessed. We could not see a septic tank lid from the aerial views of grandmother's place and I think we concluded that the lid was underground.
True, in many cases septics are a split trafficable rated concrete lid.
The lids are designed to prevent any of this weighing a 150kg+