They did have multiple emergency methods to surface. Neither those, nor a beacon help if it imploded under pressure.

Seeing that everything stopped at once, I'd bet that is exactly what happened.

Shades of the Thresher disaster. No doubt we'll hear a few months down the track that the Navy's sound receivers did actually pick up the implosion noise.

or someone makes a startup offering submarine exploration of the titanic tours crash site

Recursive Ventures

There was some speculation that any sort of implosion would've been detected by military sonar, and it hasn't been yet. I do not know if that's accurate -- just repeating what I've heard.

doesn't necessarily mean they ever tell anyone about it, it would just reveal capabilities.

Ah, I didn’t see that they had emergency resurfacing methods. I don’t have much hope for these guys.

Can you buy off the shelf emergency beacons that can surface from 400 ATM or broadcast through 13000 ft of water?

why does it need to be off the shelf, their submarine is not off the shelf. If off the shelf stuff odbly does 200 ATM, then limit your depth or mame custom ones.

Honest question, how would an inflatable something work at those depths?

4,000 meters should mean 400 bars of pressure from the water, would the thing need to be inflated at more than 400 bar?