The OBD scan: half an hour inside the car's memory
Just as important as the codes present are the codes absent. If the scanner shows readiness monitors incomplete, meaning the emissions self-tests have not finished running, the fault memory was cleared very recently, often within a few dozen kilometres. Sellers do this before viewings to blank the dashboard. It is not proof of a hidden fault, but it is proof of an erased history, and an inspector will note it and recommend a longer test drive or a re-scan after 50-100 kilometres of driving.
The scan also anchors the mileage question. Many ECUs and modules log hours run, distance at last fault, and service counters that can be compared against the odometer. A cluster showing 90,000 km on a car whose modules remember events at higher distances is a car with an adjusted odometer, full stop. This is precisely the class of finding that separates a real condition report from a seller's assurance that the car is clean.