Did they simply disappear?
That seems like the only possible answer, yes.
- If you go to Narnia and spend an amount X of time there before returning to our world, then some amount Y of time will have passed here, where the relationship between X and Y is never made quite clear except that Y << X. This is because, for whatever reason, time passes more quickly - often much more quickly - in Narnia than in our world.
- If you go to Narnia and never return to our world, then ... well, you never return to our world. If you never leave Narnia, you'll never reappear in our world. So yes, they simply disappeared.
The only other possibility is that after many years in Narnia, when they were old and their descendants were ready to rule, they might have returned to our world, perhaps returning to their original ages like the Pevensies in The Lion, the Witch, and the Wardrobe. But why would they want to do so? Who'd give up being the benevolent ruler of a happy kingdom of talking animals for a life of drudgery as a cab driver in stinking 19th-century London? Husband and wife and all their children were in Narnia; they might have left behind some friends and relatives in England, but by that time they'd surely built a new life for themselves in Narnia which they were happy enough with not to want to leave it.
Why did nobody notice their disappearance? Well, it was a pretty crazy night all round in London, with the mad witch knocking policemen senseless and so on. If the cab driver disappeared, it might have been assumed that she'd killed him; and the police would probably be much less concerned about his disappearance than that of the violent and dangerous woman. As for Helen, we're not even shown the place where she was taken from. It might have been a source of worry and bother for a while, for their friends and neighbours if nobody else, but nobody cared much about the poor folk of 19th-century London, and a couple more or less wouldn't really concern the authorities.