I was thinking the same thing. The Halo 3 terminals say what's left of Mendicant Bias is on the Ark, and he seemed to talk to us there, but who knows if 343i left any of that or wiped it all from the canon. I don't know why I didn't remember those guardian sentinels earlier-- Avateur, I think you're on the right track; if they're related to previous fiction at all, I'm sure that's what they are. As further speculation, if they're bringing back cut or semi-cut Halo 3 stuff (semi-cut because it was in that comic thing), the thing that the Master Chief saw in the desert might be a redesigned strato-sentinel. I think it's more likely to be Mendicant Bias or a War Sphinx, but you never know.