thebard3 said:
If this thing sank, it's likely to be an enduring mystery as it's almost certain to never be found. If it didn't sink, and is bobbing around out there somewhere, then the occupants will be trapped until it is found and opened from the outside. I believe in either case, it should have some kind of autonomous locators that function regardless of the condition of the vessel.
I'm just conjecturing and opining here. Perhaps someone with firsthand knowledge can pipe in.
The US Navy has a ship which could very likely find it if it's on the bottom, RV Petrel. Unfortunately, RV Petrel is currently laid up in Leith, Edinburgh, Scotland; with some damage after it recently fell over in dry dock. Petrel can operate beyond 4000m depth and would have a decent chance of locating it if USN were willing to allocate the resources for the search. I don't know the current status of the repairs to Petrel, but it could be many months before she can even leave Scotland. Petrel is the ship which located and surveyed the USS Indianapolis at 5500m, operated by Paul Allen's ocean research organisation at the time.
The Royal Navy also have RFA Proteus which is similar to RV Petrel. It's also currently laid up in dry dock, undergoing conversion and refit. Even if Petrel, Proteus, or a similar deep ocean research/surveillance vessel were on site today, that type of search could take a very long time to locate it on the bottom; the chances of finding it within the 96 hour window would be small.
If it's on the surface, there's a decent chance it will be located within the small time window where rescue would be possible. The weather and sea conditions sound like they are reasonable, and that type of maritime search and rescue is something which is well practiced and it's feasible to get a lot of resources into the area quickly.
As far as locators go, underwater is not so easy. On the surface, the standard would be an EPIRB, but normal commercial EPIRBs probably would not be able to handle the 400 bar pressure they would experience if externally mounted at 4000m depth. If it was on the surface and had a functioning EPIRB, this story would already be over; that scenario is pretty much a guaranteed rescue within about 12 hours.