We had a small fire near us that started on a clear day, but there had been lightning in previous days, sometimes they can smolder.
That does happen. But it hasn’t been stormy there that I’m aware of. And by now they would know for sure if it was a lightning strike. NOAA runs a site showing every single positive and negative lightning strike by fate and time.
Also in the forest if you go there you find an exploded tree or other evidence of the strike at the source.
But those can smolder a long time.
But I think they would know by now.