Hello Yvon - you're right, naval vessels don't have to hoist the courtesy flag ... but it's polite and fair to do it. In fact it's quite the same for merchant or civilian ship it's polite to do it but not a mandatory rule even if in some country it's very badly felt ...

As I recall, the naval vessels didn't have to hoist the courtesy flag, only merchant or civilian ship.
Have the rules changed or my memory is bad ?
BRgds, Yvon.

Arleigh Burke-class destroyer commissioned 2010.
Displacement: 9,200 tons
Length: 509 ft
Beam: 66 ft
Propulsion:
4 × General Electric LM2500-30 gas turbines
Speed: over 30 knots
Complement: 380