It is incorrect to say that kināyah has no contextual indicators at all. Custom, circumstances or another clue may lead the audience toward the connected intended sense. The defining point is that such a clue does not exclude the possibility of intending the literal sense.
Consider, “The fire in his home is never cold,” said when describing someone's hospitality. The conversational setting suggests generous hosting. In an appropriate situation, however, the literal notion of frequent cooking could also remain possible.
A clue that rules out the literal sense differs from one that merely helps identify an associated intended sense. Confusing their functions obscures the distinction.