Well - it's unlikely that the spacecraft would have gone rogue and started lengthy delta-V thruster firings...

BUT...

Safe modes often involve increased thruster firings for attitude control and whilst this isn't designed to impart a delta-v on the vehicle, it generally does under the catagory of 'small forces'. Other small forces include solar pressure, atmospheric drag etc... - and they are generally predictable if you know the attitude of the vehicle - less so if you don't. The ammount of thruster firing that has gone on though is an unknown - so it's possible that even with a tiny bit of delta V over the course of a week, the vehicle could be several hundred metres, if not several KM away from a calculated extrapolation based on the last known orbital elements.

That is why they're going to use the Optical Navigation Camera first - they know where is SHOULD be - and they can guestimate the range of likely devience from that estimate - target the ONC, see the ACTUAL location of the vehicle - calculate updated orbital elements, and then you have a much higher chance of getting the vehicle in the narrower FOV of HiRISE.

Doug