Get operating direction now
Tell the gas water-heater technician what you found and ask how the equipment should be kept off pending inspection. Follow the exact heater's shutdown directions only if you can identify and reach the controls safely.
An old mark is still worth investigating. A photograph does not establish whether it came from this appliance, when it formed, or whether the vent is now safe.
Preserve the evidence
Photograph visible marks and the vent arrangement from a safe position. Save the model, serial number, installation records, and any previous vent or combustion findings.
Note recent work, changes in surrounding space, unusual odor or noise, and when the marks were noticed. Do not touch a hot vent, remove vent sections, or conduct a draft test yourself.
Request the relevant inspection
Ask the qualified technician to assess the heater, vent system, combustion-air arrangement, and surrounding damage under the exact model's instructions and current applicable requirements.
Request the cause finding, relevant test results, and a clear statement of any work needed before operation. Have appropriately qualified providers address related building, vent, or electrical work, with responsibility identified in the scope.
If replacement is recommended, ask what equipment condition requires it and how the vent or installation issue will also be corrected. Replacing the tank alone may not resolve the actual finding.
Your next step is prompt qualified inspection and a documented operating decision. Keep a practical alternative for essential washing and bathing while the safety concern is unresolved.