Unmissable, it's a story based trophy. I'm posting this to advise how to fix a bug. It's a good idea to have a saved game shortly before you become a Spectre. If you go down to the CSEC Requisition Officer and he does not have Spectre weapons (the HMW ones) then you need to reload your saved game, go through the ceremony, and visit him again. I have verified that this unlocks the weapons even if you are much later in the game.