Hypothetically. I know nothing about this particular system but I do know for sure that such a submersible must have a surface support vessel, and would have negligible ability to travel horizontal distances... it's basically an elevator with the ability to move laterally only very slowly and probably only a few hundred meters.

So if it popped to the surface unexpectedly, it's support ship should have been able to see and recover it. That's why the ship is there to begin with. They don't do this sort of op in bad weather. And they'd have backup comms, if nothing else a SATPHONE.

The thing is unfortunately still at the bottom.