It can't be "unknown rapture" for we know it's going to come and it will be known that it occurred afterwards.
I think "secret rapture" has been used to describe that it is a signless event, the date of which is a secret. Of course, when it happens, people will know that something has happened. If "secret" does not describe the event very well, I think the word "mystery rapture" will do. Till now, the rapture is still a secret to many, but it is a mystery revealed to believers today.
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