To tell you the truth, it sounds like a simple miscalculation in their math. If they simply add the extra 13mm per second, the unexpected acceleration goes away.
It's not so much a miscalculation, as something missing in their perturbation models. I don't know if they account for relativity effects, for example -- but if they're not, they'll see an error.
It all boils down to the source of the error. It might be nothing more complicated than trajectory determination error.
That would be a fudge factor.
The problem is that current models produce very accurate predictions for, inter alia, earth satellites, without fudge factors.
Gravity models are extremely well characterized as are the effects of solar radiation and other heavenly bodies. They explain and predict the orbits of artificial down to centimeters per day, or very small fractions of a millimeter per second, in the order of 0.005 mm/sec. 13.5 mm/second is HUGH, SERIES!
Unless you tell where in the model you think the 13.5 mm/second belongs, which parameters you care to modify and how your results are consist with satellite observations, your suggestion is useless.
It is provocative that all the anomalies occur on hyperbolic orbits, orbits for which the smaller body has escape velocity. Sir Isaac would be perturbed, since his laws, corrected by Dr. Einstein, used to work in both cases.
I’m thinking there is a hockey stick in there somewhere. :)