I think I may have found out why my maternal great grandparents (George Hutchings and Elizabeth Sarah Edwards) seemed to totally disappear after 1881. I sent away for a possible death certificate for George and when it came back it revealed he had died in the Maidstone Lunatic Asylum in 1898, so I checked the 1891 census for the same location and discovered that only the initials of inmates were included in the census, hence his not showing up on my previous searches. I found a suitable candidate there, so that took care of George.
As the two of them had never actually married, I decided to resume my search for Elizabeth Sarah under her maiden name, as I had found nothing under the name of Hutchings after 1881 (despite her appearing in previous censuses with that name). I still could find no sign of her in the later censuses and no death notice either, so I guessed she may have changed her name and the most common reason for that with a female is, of course, marriage.
I then found a marriage to a James Eaton Barty, a widower, which fitted the bill – her name, place of birth, date of birth and name of father all fitted and she was listed as a spinster, which technically she was, despite having lived with George for many years and bearing a number of children to him (including my grandfather Aaron Edwards).
So, that is how two people can apparently disappear from the records – and I think that mystery is solved.
I will put all this on my growing family tree on Ancestry.com and hope that someone there will recognise James Eaton Barty (he does appear on some of the family trees there) and confirm my deductions.