As a veteran of more than a few of these types of cases when I was in Uncle Sam's Confused Group, please allow me to conjecture...
Judging by what I've read about the boat, I would venture a guess that it would have been Boston Whaler type fishing boat. Low freeboard, somewhat unstable in any kind of weather other than FAC, and fiberglass. There would be almost no radar return off such a vessel. Even fiberglass sailboats, with that nice tall mast will give almost no return, so most responsible boaters will run some type of radar reflector to the top of the mast to give radar something to ping off of. This type of boat won't have such a capability, so essentially, they're invisible to large traffic.
More than likely, the only communication device they'll have onboard will be a cell phone. You wouldn't believe the people out there that think that a radio is now a superfluous, unnecessary object. That's a big problem these days, especially with the new Rescue 21 system that's coming online. It can triangulate even a momentary click of the radio mic, but does not have this capability with cell phones. And phone use is notoriously spotty on the ocean. Even among the San Juan Islands when I was stationed in Bellingham, WA, there were large areas with no cell coverage.
More than likely no EPIRB either, for the same reason. Why spend a few hundred bucks when help is just a phone call away? Other signal devices such as day and night flares, mirrors, and dye packs; ditto. Again, the over reliance on cell phones.
The average (and I stress average, there are a few sailors/boaters out there that learn them) know diddly about the Rules of the Road. Private vessels over a certain size (45' if I remember correctly, but not 100% sure) are required to carry the book, but I doubt many people have cracked it. It's boring. I still have my copy from when I was studying for the NAVRULS test when I was in the Guard, and I hated having to study it. This is why states are starting to license boaters like cars, but that's an uphill battle.
Hopefully, they find them alive, but I'm not holding my breath.
Hope I'm wrong...