Mary Dejevsky: You don't wash a car on a Saturday morning

Keep your eyes open, by all means. But think twice before shopping your neighbour

There was a time, before the programme became as contrived as it is now, when I enjoyed watching Wife Swap. The appeal lay not just in the prurient glimpses behind the net curtain of other people's lives, but in the subtle give-aways about class, wealth and education that lurked in every clip.

There was a third attraction, too: the infinitely mysterious dynamic of other people's marriages - and the thought of how many tiny details evolve into one all-embracing normality. How could someone else, I liked to muse, figure out in a week exactly how and when my husband liked his tea, how the heating should be programmed, and the disconcerting reality that any ice-cream in the freezer will be gone tomorrow unless you expressly forbid its consumption.

He would doubtless have a similar list of eclectic instructions about getting along with me, which might include "don't talk to her before she's had her coffee which, by the way, she likes just so". And "don't dare tear anything out of the paper before she's read it". Our respective substitutes would have to master hundreds of these minutiae before they could even approach doing a reasonable job.

Perhaps we are both just flattering ourselves about our irreplaceability. Somehow, though, I think there is more to the observation of mundane details.

Consider for a moment what some residents of Houston, a commuter village in Renfrewshire, have said about their neighbours - two men believed to be among those arrested in connection with the car bomb at Glasgow airport.

One observation, which might seem banal, is that one of the men was seen washing his car - a large 4x4 - outside the house at 8am on a Saturday. You do not have to live in suburbia very long to know that cars are washed in late morning or early afternoon, not at a time that would be early even for breakfast at the weekend. There's no reason why someone should not wash their car at that time - this is not the United States, where a community by-law might well outlaw such non-conformity - but it is just not what most people do. People notice, and remember.

Another observation is that the two men seemed to drive a variety of cars. It is not uncommon for suburban households to have two, or even three, cars. But if they do, the cars will usually be parked on the driveway or outside, and each will mostly be driven by one person. Cars tend not to be pooled, except in an emergency. The notion that these neighbours had four or more different cars at their disposal - cars that were not regularly parked outside their house - set them apart. Again, it might have been remarked on at the time as curious rather than malign, but it is something that people remembered.

A third detail is so typically neighbourly as to elicit a wry smile. On one occasion, a car was left parked outside the men's house with its headlamps on. Someone helpfully knocked on the door to let them know. Instead of being met with heartfelt thanks, personal introductions and pleasantries about the price of car batteries, the neighbour felt that his intervention was unwelcome. It was not the expected response; it lingered in the memory.

Something similar could be said of the late-night visitors in expensive cars. Junior hospital doctors working shifts might indeed leave and return at strange times of the day or night. They might invite friends back with them. And there have been occasions when, after misjudging the distance or getting caught in an epic traffic jam, I have driven to a friend's house in the early hours.

But in places where few car doors are heard closing after 10pm during the working week, such things are noticed. If they happen more than once or twice, and if the cars are easily recognised, such minutiae are filed away in people's memories.

It is only with the benefit of hindsight and a dramatic prompt, such as the Glasgow bomb, that people unlock their memories and impose a bigger picture on the fragments of their observations. Until then, in their very British don't-like-to-pry way, they have contented themselves with thinking the neighbours just a little odd. That is normal.

The risk now is that we will all be twitching at our curtains, mobiles pre-set to dial 999 the instant the man next door decides, just for the heck of it, to wash his car at 8am. That does not make him a criminal (unless a hosepipe ban is in force) nor a terrorist in training. Keep your eyes open, by all means. But think twice before shopping your neighbour. Distinguishing the normal from abnormal is not as easy as it looks. One episode of Wife Swap will be enough to establish how strangely other people live their lives.

m.dejevsky@independent.co.uk