So, I've had this weird thing for a while now where something appears in the floor at the entrance to my dormizone:
Sometimes it simply appears as above, but sometimes it doesn't. When it doesn't, it allows me to perform a stealth attack which spawns a frontier grineer:
This then causes the thing in the first pic to appear again. Whilst I don't have a screenshot of it, I remember that one time it appeared out of the floor, it seemed to be the model for the shield of the Tenet Agendus, if that helps.