The Gravemind appears to be capable of (whenever it becomes large enough to be "a" Gravemind) somehow "remembering" everything that prior instances of it did, said or encountered.
The one on Delta Halo (Installation 05) stated that it had been "listening" for millennia.
Perhaps the one that we encountered on High Charity, the Ark and Installation 04 mark II, we're just an "off-shoot" of the one that was on Installation 05. That Gravemind may still be there. I don't see how something that massive could have easily transported itself onto High Charity (even using In Amber Clad).
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a Gravemind would be a HUGE pain to move. If all my studies are correct then the gravemind is simply a form of mycelium. If he is in fact a mycelium, this would explain not only the flood's extremely parasitic nature, the polypore mushrooms growing around the gravemind and the fact that once it "Moved" from installation to High Charity it lost some mass and became once again a wriggling mass of tentacles.
Mycelium is like the root system to mushrooms and other fungi, mycelium cannot "break" apart, it is one mass. However, Where mycelium cannot divide, mushrooms CAN drop spores, these spores spawning new masses of mycelium... It is still very possible that the flood remain there on that installation.
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As you can see, the chances are horribly high that there are still flood and master chief will have another game. More info will be posted later