I'd be looking at all the obvious things first, loose gubbins in the boot. Anything moving about under the boot floor? Even the boot lid itself, (although unlikely) could be a source of knocking, something like the wire loom not fitted and tied down sufficiently.
The comment on modes is valid, is it the same in all modes? What if there is someone sitting in the back, does it change? Can they sense where it comes from? Like in the car, under the car.
Many times something like this has caused 'scratching of heads', when it is something obvious, once tracked down, but not necessarily the suspension. Even how one of the rear doors shut, most unlikely, but could be the issue. Exhausts and heat shields are clearly an obvious source, but not always easy to see how from a static test.
If the issue is identified as being in the suspension, it still may need a bit of head scratching to find the source.
As to identifying the 'physical' adaptive suspension, swing the steering to one side and you should easily see the proportional valve sticking out from the strut with wire attached.
HighlandPete