Update: I don't know about as a reward, but they can sometimes be items advertised in prospecting missions. I actually found a Mystery Part the following week from this comment. Normally, you have to hand over these types of items to your client, but if you sprint/run over the item, or you have a full inventory, you can swap the item on the ground into your inventory and the mission will not detect it.
The mission will be closed as successful if you escape/clear the dungeon, and you will be able to keep the item.