Trace the deposition pattern

Compare the highest or earliest wet point with the broader dust-covered area below it. A narrow origin feeding a larger stain suggests a source path, while a uniform film on several unrelated surfaces may suggest external deposition. Wind can move fluid sideways and gravity can move it downward. The dirtiest location may simply be where fluid collected rather than where it escaped.

Separate old residue from active recurrence

Use known maintenance history and subsequent observations to distinguish a one-time spill from continued accumulation. A stable stain after servicing is different evidence from fresh wet material repeatedly appearing at the same boundary. Any cleaning used to improve visibility must follow approved instructions and preserve necessary evidence. Do not use hands to search for pressurized leaks or open a fluid system merely to compare the stain.

If the oily patch returns at the same upper boundary after authorized cleaning, that recurrence supports further source investigation. If only an old lower stain remains, the evidence is different. Neither case should be evaluated solely by the total area holding dust.

Ask for a source-based inspection

Provide the suspected fluid type only if known, the observed path and any independently recorded level change. Ask the service specialist what examination will distinguish external contamination from leakage and whether the affected component needs restrictions. An unchanged reservoir reading does not necessarily resolve a small leak. No MARSTON fluid-system arrangement is assumed; these questions apply only where the installed equipment actually contains the relevant fluid.