The sea floor in the area where the debris was discovered is around 3000 metres and flat and featureless.
3000 metres is well within range of ROV's.
Here is a marine chart image of the seabed from a screenshot.
http://pprune1234.smugmug.com/Pprune...reckage-X3.jpg
Thanks to PJ2 for taking the time to explain to me how to post it.