I'm on my second playthrough and I'm realizing I really don't have any idea how the having of weapon mods works. The basic level of "find a mod on a weapon and now you can put it on yours" makes sense enough, and I know obviously that some mods can only go on certain weapons, even if sometimes it doesn't make sense (looking at you, scopes), but it still leaves a lot of weird inconsistencies. Sometimes I'll pick up a mod that displays as new, but not see anything new in the backpack, sometimes a mod I know works for multiple weapons will show up as an option for one but not the other. Sometimes an mod I had just been using won't show up while on a full workbench, but when I use my backpack immediately after it does. I think I've had mods just flat out disappear.
I do think there might be some kind of hidden "inventory" of mods that's kept, where instead of "unlocking" them you have a limited amount to put on sunderly weapons or something, in which case it'd've been nice for them to indicate that in some way but, it still leaves some things kind of unanswered? What am I missing?