Update:
Picking up the car from Nissan tomorrow, it has been fixed under warranty. The issue was a gasket that had blown out, in the rear timing chain cover, which was replaced. It caused the drop in oil pressure, not enough to cause anything too serious but enough to notice it with the AEM gauge. Without the gauge, it would be impossible to tell anything had happened unless more of the gasket blew out, to the point where the pressure was low enough at idle to trigger the low oil pressure warning light.
Apparently this has been seen before on Murano's by the dealership, and likely on Z's before as techline immediately suggested this as the cause of the issue to the dealership. No idea what caused it to happen, at least it is sorted out though. If it had been out of warranty, the expense would have been significant - about 20 hours labour!
Pic of the old cover and where the gasket was gone out:
