The damage diagram is not always accurate
Suppliers use a schematic with marked existing damage. It is often out of date, particularly on cars that have been rented many times.
Treat it as a starting point to be corrected, not a record to be accepted. Compare it against the actual car panel by panel.
Getting something added properly
Write it on the form
Anything you find must be added to the handover document and initialled by the agent. Verbal acknowledgement has no value at return.
Photograph the marked form
Once the addition is made, photograph the form itself alongside the damage. That links the record to the car and the date.
Ask for a copy
Leave with a copy showing the amended damage record, not just the rental terms.
If the agent will not record it
Some counters resist adding minor damage. If that happens, photograph the damage clearly with a timestamp and email the supplier the same day, describing the car by registration and the location.
A dated email sent before you drove any distance is strong evidence. Keep the reply.
Damage found after leaving
If you notice something once you have driven away, report it immediately rather than at return. The longer the gap, the harder it is to argue it was not yours.
Photograph it, note the time and mileage, and contact the supplier the same day.