Perhaps the Polar Lander and Beagle did!If there were a lot of rocks, small Planet X that produced an equal and constant acceleration in both spacecraft. then I would have thought they would have bumped into one by now.
All other considerations aside, the maintenance of a nominal orbit depends upon the momentum of the spacecraft and assuming that NASA is able to program and measure that accurately the problem is most likely one of defects in their calculations. Few if any scientists acknowledge that the traditional formula for centripetal force has an error factor in it which increases as the orbital period decreases. This is what concerns me with the DAWN satellites which NASA hopes to insert into orbit around small asteroids.
And NASA may like the publicity that some apparently inexpicible phenomena can bring - the public love a mystery.