If it happened when you had the lights apart then check there aren't any extra wires put in by Mr Previous Owner - maybe for a live feed for a camera for example that you could have connected in place of the correct cable.
Failing that a battery disconnect costs nothing and is easy enough to do so worth a try. Only other things I can think of is a faulty Light Switch Module or faulty cabling between the LSM and the number plate lights where cables have rubbed through the insulation and you are picking up another live feed.
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