In days gone by it was accepted normal practice at a local school to leave fuel injected aircraft running, with the clutch disengaged for quite long periods between flights, blades going round but not under direct power. This was because they are not easy to restart when hot. Engine running refueling was also normal practice.
It was not unusual to see a couple of unattended, running aircraft. The type in question did not rely on friction to hold the cyclic central.
The practice has now stopped I believe at the request of the airport operator. They must have done a risk assesment and didnt like it. As far as I know it never caused any problems. It just made one careful on the apron.
I'm sure many other PPRuNers will have seen this too.