What is the being signaled when you hear 5 short blasts of another vessel's horn?

If a boater sounds five short blasts on a horn, it represents danger, or that the boater does not understand the approaching boat's intentions.

If the boater sees another boat approaching him, and he suspects that something fishy is going on, or he doesn't understand what the other boat is trying to do, he will sound five short blasts on his horn, in order to show that something is wrong. So, the correct answer is danger or doubt.