I get the impression that the 3 divers were leaving the wreck. As they neared the exit, the last diver (Tin) might have had some problem. Steve signalled Chow and then returned to assist Tin. Chow went to follow Steve. At that time the visibility had deteriorated to a dangerous level. Chow couldn't make contact with either of the other divers and aborted... leaving the wreck by deploying a safety line.
I doubt anyone will ever know what exactly happened with Tin and Steve. We could theorize that Steve was trying to assist Tin, when his own air ran out. As an experienced instructor, he would have been under considerable self-imposed pressure to 'look after' a diver that he would have seen as his responsibility... and possibly pushed his attempted rescue too far, or then got disorientated/trapped/lost.
Tin had air left when he died, which opens up a lot of possibilities; as just 'being lost' wasn't the only factor in his death. Medical complications or panic must be seen as primary possibilities. Tin's remaining air may also indicate that he died before Steve. If Tin died early in the incident, it would explain why he air remaining and also why Steve didn't. It would also fit with the fact that Chow had to abort and surface (I assume, due to diminishing gas).
I don't read much into the fact that Tin's mask and reg were displaced when they recovered his body. It could have happened before (contributing to) the incident, it could have happened during the incident (maybe indicating panic), but it could also have happened post-mortem (if Steve was trying to recover him).
The 'general' protocol for zero vis is that the team groups and exits whilst maintaining touch contact and communicating via tactile signals. If the team is separated, then the missing diver drill is used. That basically consists of shading lights and looking. If that isn't successful, the remaining team will check their gas supplies. Their gas will determine the extent of their search. To search, they will mark the line and then quickly search back down the line (if the line is still deployed behind them or permanent). If that isn't successful then the team will deploy a safety line off the primary line and search using that.
The primary line is left in place, until the team is reunited and ready to exit. If the team isn't reunited, then the line stays (with marker) to enable a more rapid rescue/recover and also to give the lost diver some chance of still finding their way out. Divers may also (ideally) leave their search line in place. There may be reasons why that might not be considered the best option. Any lines deployed in those circumstances will always be marked to indicate the direction of exit.
Whether the whole team conduct that search/es, or whether one member of the team does it alone isn't standardised amongst the community.
My personal preference is to have a single searcher (me). The remaining team member (team max of 3) might wait where the line was initially marked and the primary/safety lines are joined.
It is possible to for each remaining team member to deploy their own safety line and run simultaneous searches - the benefit being more distance/options covered. However, it really does depend on the visibility - as coordinating separate searches in low/zero vis may well pose more problems and risks than the benefit it provides.
It really depends on the experience of the team. An experienced diver leading two less experienced divers may choose to take more personal responsibility for the search and prefer that his remaining team member was static (my preference). They may even escort the remaining member out of the wreck and leave them at the exit point, whilst they returned inside to search.
Ideally, the precise protocol to be used would be communicated and understood prior to the dive.
From the reports thus far, it's hard to work out what protocol the divers concerned were using. We don't know if Steve communicated with Chow to stay static, to exit the wreck or to return with him to assist/find Tin. If he did intended for Chow to return with him, then he should have waited for Chow to catch up and they should have gone back together cohesively and in constant contact.
We also don't know whether the team was on the permanent line, or whether they had deployed their own guideline. If they did, were they reeling out? If so, Tin (3rd diver) would have been operating the reel. That makes me think they were on the permanent line (or in an area where line is not typically needed) - otherwise Chow and Steve would have been able to follow a deployed guideline straight back to Tin.
Anyone who dove with Steve on similar dives, might care to share insights on what protocols Steve usually briefed/taught/followed?
