re: Maybe I'm missing something.
Heat (and reflected light) radiates irregularly from a spacecraft. This results in a net thrust in some direction or another. It depends a lot on the spacecrafts orientation in space. The Planetary Society is studying this to rule it out as the cause of this anomoly. I recall (but will not research) that some spacecraft used changes in orientation to effect a small course change rather than using their engines. This requires a good understanding of the conduction/reflectivity profile of your vehicle. I believe that Messenger did this but I could be wrong.