The secret "statuses" that companies grant to a lucky few frequent flyers
They are not officially published on the elite status pages of individual loyalty programs, do not appear in brochures or [...]

They are not officially published on the elite status pages of individual loyalty programs, do not appear in brochures or graphics, but exist and allow holders to enjoy special services and treatments.
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They are not easily given and it is not even how much you fly that is the key requirement, more of a way in which the companies want to let the traveling staff know that there is a passenger on board who is different from the others and make sure that throughout his or her experience, on the ground as well as in the airplane, the passenger is treated "with kid gloves."
Air France KLM's Club2000
Obviously I start from this because. I can say firsthand that it exists and not from hearsay. The Flying Blue program has two "secret" levels in addition to the Club2000/Skipper Club is also told of the existence of Hippocampe status, If the first one in Italy we have it in less than 60 and there are about 2,000 in the world, the second is for the true world elite.
At the level of treatment, I must say that I have seen the difference between flying as a "simple" Platinum and flying as a C2000, let alone how those who have the most secret levels of the Franco-Dutch loyalty program are treated.
Delta360
Staying within the Skyteam alliance, Delta the most profitable of airlines has a level called 360. This status is invitation-only and gives its holders unique experiences like the one my friend had Christian who was welcomed to the track by two Porsches, one for him and one for luggage.
Emirates iO
I who am a bum Emirates first class I can only see it because of the points, but those who can afford to spend at least $1M with the Dubai-based company might have access to the "iO" card. A stauts similar to the Hippocampe mentioned earlier that opens the door to royal treatment. According to what we read online this card lasts for 2 years and, if not renewed by the company, guarantees a soft landing at Platinum the highest level of EK's FF.
The benefits go beyond the flight as the lucky chosen ones also have open doors to Emirates-sponsored sporting events around the world.
Other secret fidelity levels
There is the Pandion Club at SAS, but I have never met anyone with this card. I have read a lot on the net about the Qantas Chairman's Lounge, but surely there are at least a couple of other levels that I don't know about (if they are secret, there must be a reason). What is certain is that in Europe we have cards that are not secret but certainly exclusive as the few, very few, Lufthansa's HON Circle or British Airways' Concorde Key holders. In the latter two cases, however, all you have to do is fly, lots and well, and you will be able to get into heaven even without an invitation.