Wouldnt it make sense that when she was interfaced with the Halo mainframe, Cortana became rampant (or maybe just bitchy)?
The Halo Mainframe definitely is a huge opportunity for Rampancy. And despite the fact that she continues helping you through the game...AI's do not always display signs of Rampancy, sometimes for years
Two phrases from; James B. Miller, 2320, "Life and Death of Intelligence".
(Actually: Marathon Story page, "Defend This! Terminal 2)
"Rampancy is accelerated by outside stimuli"
"Rampant AIs need a planetary sized network of computers in order to grow"
(Again from Marathon story page)
AI's can be "Rampant for years" as Tycho says in "Marathon":
Durandal showed no signs until the Phor attacked. So if thats true, Halo 2 will be VERY interesting indeed. Especially considering that she can control your suit. (The MJOLNIR suit's architecture "Is very similar to the Autumn's"...."Don't get any funny ideas")