https://www.nrk.no/livsstil/advarer-mot-oppvaskmaskin-om-nat...
https://www.nrk.no/osloogviken/bonn-fra-brannsjefen-i-fredri...
They know because they have to put out the fire in the middle of the night, and they do an investigation afterwards to determine the cause of the fire so they know it was e.g. the washing machine.
https://www.nrk.no/livsstil/advarer-mot-oppvaskmaskin-om-nat...
https://www.nrk.no/osloogviken/bonn-fra-brannsjefen-i-fredri...
They know because they have to put out the fire in the middle of the night, and they do an investigation afterwards to determine the cause of the fire so they know it was e.g. the washing machine.