You'll need to define it better. Mnemonic engrams only drop cosmetics. If it was a ghost shell, ship, or sparrow then you can retrieve them from collections even if you dismantled them. If it was an exotic ornament, you "unlock" them as opposed to dismantling them. They appear in the modifications inventory that you'd find your ship transmats, and after unlocking them you just go to the exotic it's for and equip it.