About a decade ago, I worked at a beige box store. Someone brought in just his hard drive, and asked me to recover all his files, as his thesis was on it. So, I ran our recovery software (can't recall what it was) and found the drive was completely blank. No system files, no data, no temp files, absolutely nothing. The customer didn't seem upset or irritated at all, and thanked me, paid his bill and left.

At the time, that puzzled me a bit, but now it occurs to me that he may just have been making sure his data was destroyed.