I've known this happen with a washing machine when it stopped being used daily.
The electrically-operated fill valve contained rust and scale residue from recent plumbing work. This prevented it sealing shut correctly and water dripped into the tub.
When the washer was being used frequently, the water just got used as part of the wash cycle, and the tub was pumped empty at the end.
When the washer was no longer used, the drip continued to fill the tub until it overflowed.
You might detect this by giving the tub a bit of a turn by hand. If there is much water in the tub, you will hear it splashing about.
Turning off the water supply valve under the sink will be a temprary workround. I have an idea a callout to change the fill valve will cost around £50-£100. Bosch valves are non-standard and more expensive than most. It can be a DIY repair if you are handy.
It was the washing machine as expected. However, they were not 100% sure of the precise cause. Anyway, I got a new washing machine installed for free, as the solution.
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