I Picked up "Mark of the Colliders" a couple of times (once from a Prime Engram and once from a Legendary drop) and I don't see it in my collections when I look at the Armor category (checked every section).
ETA: Found another Titan mark that doesn't show up in collections - "Ere The End".
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Each class has (I believe) two or three class items that don't show up in collections. I don't have a list of them unfortunatley.