Five short horn blast from another vessel?

From the USCG Navigational Rules of the Road (International and Inland.)

Rule 34

Maneuvering and Warning Signals

(d) When vessels in sight of one another are approaching each other and from any cause one fails to understand the intentions or actions of another, or is in doubt whether sufficient action is being taken by the other to avoid collision, the vessel in doubt shall immediately indicate such doubt by giving at least 5 short and rapid blasts of the ships whistle.

From the USCG Navigational Rules of the Road (International and Inland.)

Rule 34

Maneuvering and Warning Signals

(d) When vessels in sight of one another are approaching each other and from any cause one fails to understand the intentions or actions of another, or is in doubt whether sufficient action is being taken by the other to avoid collision, the vessel in doubt shall immediately indicate such doubt by giving at least 5 short and rapid blasts of the ships whistle....