Same old story used to do the rounds suggesting that someone had landed on an unmanned oil platform, helicopter disappeared due poor friction, another one landed, pilot went down other steps to toilet.........etc etc.
Actually, one of those is not so much an urban myth, and I'm sure there are one or two other Ppruners out there who might be able to confirm this one.
Many moons ago a pilot left his helicopter running but unattended on the deck of an offshore installation (not sure whether it was manned or unmanned, bu I thin it was somewhere in the gulf (not of Mexico!)). The helicopter 'took a walk' across the helideck and disappeared over the side ..................
As for the original story on this thread. I have heard this particlar story in a number of different ways. One being that two overseas officers were posted to a UK military base. Both when flying, one landed for a call of nature and the heli rolled down the slope he had landed at the top of. Colleague landed to check his mate was OK. Matey takes colleagues heli, heads back to base and was found later reading a newspaper in the officers mess. Looked very surprised when asked what had happened etc etc....... Therefore tend to agree that it has to be a myth.