Used in conditional expressions, often as part of もしかして or もしかすると, meaning 'perhaps' or 'if by any chance'.
Perhaps he has already arrived.
In case it rains, let's take an umbrella.
もしか is another bound adverb that, like ひょっと, mainly appears combined with すると/したら/して to hedge a guess, though もしか sounds slightly more common in casual speech.
Derived from もし (if) with the addition of か (question marker), forming a tentative conditional expression.