There is no way to detect HD contents past the current write. It has never been demonstrated that anyone has done it, and data recovery companies will not recover your disk if you accidentally erase the blocks. The misconception that you can do that comes from a misreading of a paper by Gutmann that only applied to an obsolete disk recording method in the first place.

The reason people use methods other than 1-pass erasing is because of marketing thinking that more passes looks better on the product features. Or because the IT people enjoy buying heavy machinery so they can destroy stuff with it.