A rhetorical question, right? Our world is full of immoral mechanics and car salesmen who know it is far cheaper to hide faults than to fix them.
My 87 DL came to me with the CEL bulb not doing a proper self-check at start-up. The bulb had been removed of course. I installed a new bulb and was able to fix the faults. I later found the old bulb under the passenger seat when cleaning up the interior. I bought the car from the original owner but of course he claimed to know nothing about it.