I was once tasked to carry out a post maintenance air test on a Blackhawk (obviously not in NI). Having signed the tech log we walked out to it. As we got closer, we realised that the APU was running and assumed an engineer was still carrying out some unknown check inside. However, the aircraft was totally unmanned!
We shut it down and went back to the engineers’ office to find out what was going on. The duty engineer had previously been carrying out some systems checks and subsequently seemed to think that it was perfectly acceptable to leave the APU running unmonitored and go inside. Discussion ensued. He (and his line boss) were mistakenly under the impression that the APU was fully automatic and would shut itself down in the event of any problem. We pointed out that would only shut down in the event of it sensing its own oil pressure failure but NOT if it caught fire. They went rather sheepish.