I serviced a HP machine that had on it’s service partition, a disk check tool (among other things). Suspecting a failing hard drive, I ran this checker on boot from the recovery partition and it passed with flying colors.
Continuing to see evidence of hardware failure, I cross-checked these results with smartmontools and got a very different story. Even the short test was unable to fully complete due to errors and a plethora of bad sectors, later confirmed by chkdsk.
The moral of the story: use multiple overlapping tools if something is telling you things are OK even though the evidence suggests otherwise. You don’t know how these applications were put together, what they actually do, or if some of them were ever intended to do anything other than give you a warm and fuzzy feeling.
Trust, but verify.