Around ~35 minutes into the reveal, we can see Zavala looking at a giant structure in the distance on Titan.
http://i.imgur.com/J7cSd1L.png
Originally, I thought this could be the Dreadnaught. But due to the colossal size of the ship, it isn't plausible (unless it broke apart). However, Oryx had part of his fleet decimated when the Queen attacked. It would be entirely possible for one of the Hive warships/tombships to get caught in Titan's orbit and eventually crash land into the vast ocean.
http://i.imgur.com/Rm8nufC.png
What do you all think?
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Agreed. In my opinion this has always made the best sense for explaining why there are Hive on Titan.