Aftermarket device wired into the circuit

Something added to the vehicle and connected into an existing circuit — an alarm, a tracker, a dashcam, a towbar module, a remote starter. The module detects a load, a voltage or a signal it did not expect and reports the circuit, which is functioning exactly as the standard designed it.

Splicing is what causes most of it. A device tapped into a signal wire loads it; one tapped into a switched supply draws current the circuit was not sized for; and an insulation-displacement connector crushed onto a wire damages strands and creates a high-resistance joint that fails months later. A fault dating from an accessory installation is this until ruled out, and the installation is rarely mentioned unless asked about directly.

Parasitic drain is the other common presentation — a device that does not sleep flattens the battery over a few days, and the resulting low-voltage codes appear across many modules at once. Ask what has been fitted and find it physically, since the wiring is usually behind the dashboard and not on any diagram. Disconnecting the device and re-testing is the definitive check and settles the question in one step.