A "mystery" is a "secret hidden in the past, but now revealed."
On that definition, the rapture is indeed a mystery.
Can't we also say that it's a "secret" in the sense that the date is unknown and that it is a signless event?

In that sense it is best to say the Unknown rapture concering that it is unknown to men. but to call it a secret rapture goes beyond the logic of it. it will be know secret when it happens to all involved including the unsaved masses.

so an unknown date for the rapture can't really be a secret rapture as we all know the rapture will take place but at a furture time we know not. a secret rapture would entail a cloaked rapture that no one experiences except the saved. that would not be the case in a real sense as we have stated when 900 million people and countless graves and Urns go empty it will be known that something took place and has caused great concern to everyone. but the antichrist will be there to give the answer.

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.