I still don't understand why the boiler would ignite when both HW and CH were in the off position. Even if it only ignited for a few minutes.
Not sure that your aged boiler has it, but on modern boilers they are equipped with frost protection, and IIRC an auto run facilityto to try to prevent anything seizing up from non-operation.
My money is on the no flame sensor - it will either be thermocouple if old style, or an ionisation detector if 'modern.'
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