I strongly suspect that they were in a "stealth" mode with no radar or radio emissions.
But, they should have been listening for the active radar on the freighter, if it was turned on. I suspect it was turned off.
OTOH a definitive answer may very well reveal too much information about USN operations, and the public will probably never get one.
Good point. I was on a merchant ship transiting the Malacca Straits, very dark night, and was on high alert for pirates.
Had a small blip on the radar that showed a small target was going to pass very close to us, no navigation lights.
I waited till he got close and zapped him with a powerful light that must have looked like lightening to him. Singapore Navy gunboat.
I seriously doubt they were running without navigational radar, that would be a violation of IMO protocols.
Even cheap maritime navigational radar sets have an interference reject mode that filters out most interference from other radar transmitters. Interference reject is sometimes useful in harbor when dozens of confusing strobes might flood the screen. With interference reject off, the operator will see a line on the screen indicating the bearing to the transmitter, usually another navigational radar, but no information about range. Still good to know.
Everything so far seems to indicate extreme negligence and dereliction on the part of both crews, but far more serious on the part of the Naval vessel, which had far more personnel and resources.