In some so-called impersonal verbs the person is left unexpressed because the actor is understood or implied in the action. So
a. In expressions of natural phenomena originally viewed as produced by a divine agent: βροντᾷ
b. When the agent is known from the action, which is viewed as alone of importance: σαλπίζει
Herbert Weir Smyth [n.d.], A Greek Grammar for Colleges; Machine readable text [info] [word count] [Smyth].