There is a combination system in place that which can be used to unlock hidden attachments. I will not tell the combinations, but I will say their components should be fairly obvious. There are two secret attachments.

Note that when you have met the criteria for a secret attachment, it can be found in the Perks slot.

(alternatively you could just extract the addon and look in the code but where's the fun in that? )

Happy hunting.

EDIT: Since it's been said about a million times in Arctic's personal server, it's only fair it should be written here. The components for secret attachments are the junk attachments that only hinder your guns. If that's not obvious enough, read the description of some of the attachments. That's a very generous hint.

SECOND EDIT: I will delete any comments that spoil the combinations. I actually read all the posts you guys send. This thread is to make server owners aware of these attachments' existence, not discuss how to access secrets.