They are a reason to look for a history of moisture, not proof that the grinding chamber has rusted through. Water can carry staining from another fitting down the outside of the appliance.
What the observation tells you
Photograph the pattern before wiping accessible surfaces according to the care instructions. Do not sand the casing, paint over corrosion or loosen a cover to inspect behind it. Repeated dampness matters even if no drip is present when you look.
A useful next step
Tell the engineer where the track starts and whether it reappears after sink use. If the body or mounting looks damaged, keep the appliance unused pending assessment.
Manuals and supporting guidance
Model-specific instructions take priority. These references support the practical principles discussed here; they do not diagnose your appliance.