Assuming it was switched on
I would always assume that a combusting electrical device was switched on at the time, as using electricity generates heat. Occam's Razor: the hotter something is, the fewer additional factors are required for it to reach combustion point.
My own smartphone has at times become quite warm during periods of intense use, but never when it's just sitting in my shirt pocket. The guy was using it, no need for facetiousness.