I have only seen this happen one time. I can tell you what happened, but it is not a thing to inspire confidence, not because I think the crew did anything particularly wrong but because I don't know what they could have done better.
It happened on a shallow reef in Key Largo. It was the second dive of an afternoon trip in an area with a lot of boats from different dive operators. The pre-dive briefing told us we were to be back on the boat in 50 minutes. About an hour later, as everyone was packing their gear up after the dive, a roll call was taken, and a buddy team (husband and wife) was missing. The DM made absolutely certain that they were missing, and we just waited. Because it was a shallow area, we knew that even with AL80s someone with a good SAC rate could still be diving. They could have gotten lost, but where would that be? They could have gone any direction. All eyes scanned the water for bubbles as we finished packing, but if they had made a navigational error early in the dive they could have been far, far out of sight.
All the other dive boats in the area had left and everyone on our boat was done packing gear about 20 minutes later when bubbles were spotted. The DM donned fins and snorkel and sprinted out to them. When he reached the bubbles he raised his head to give the OK sign and then free dived down to tell the couple to get back to the boat. When the couple got back on the boat, the captain asked them (with surprising politeness) if they had understood the concept of getting back to the boat in 50 minutes. (They were German tourists.) They blithely said that, yes, they had understood the orders, but since they did not carry either dive watches or computers, they had no way of obeying. They were totally undisturbed, as if the captain was a fool for expecting them to come back at a specific time when they had no way of telling time. Neither did they seem disturbed by an entire boat full of divers who were already totally packed staring at them in disbelief.
During their absence there was a total feeling of helplessness on everyone's part. I don't know what they would have done if things had gone further. The only thing I can think of off the top of my head is that they could have contacted the other boats in the area and asked them to have a look for bubbles before they left.