Wired On Friday/Danny O'Brien: When people express concern about the privacy-violating nature of new technology, they're generally thinking of far-off future violations.

When Google introduced a new hotmail-like Web mail system, the licence suggested that their computers would store your mail forever, even after you'd deleted it from their site.

Naturally, many worried that this data could be uncovered and used maliciously by either Google or other snooping third-parties. Oh yes, they say, we're sure Google is playing nice now - but what happens if Google turns evil, and still has our data? Or what happens if the government turns malicious, and can use the DNA database it has collected on its own citizens?

The truth is, however, that it may not be future leaks of information that will prove to be the strongest forms of privacy violation. It may be the scads of data that all of us have already leaked. In particular, your family photos, published online for all to see, may reveal more about you than you ever thought possible.

The truth is, there's already far more data out there than any current computer system can cope with. (Indeed, many security analysts, faced with government demands that the security services be provided with more information to fight terrorists, have pointed out that they can't even cope with what data they have already - and that drowning them in yet more extraneous detail may just be making their job harder.)

Often the data that's easiest to process, because it's readily available and unconnected to any privacy concerns or complex data protection safeguards, is the stuff we leave around us.

Take one of the first truly shocking privacy invasion in many digital veterans' lives. Old-timers on the Net used to communicate on a system called Usenet. Usenet is a distributed Net forum that dates back to 1979. For many years it was the primary arena for long-running public discussions by young engineers and students - young people who went on to be prominent figures in the growth and development of the Net.

As in most forums, the conversation was often heated and sometimes very personal - but since messages only stayed current for a few days, old battles were quickly forgotten. After the Web arrived, Usenet's popularity declined.

That was until 2001, when Google placed a searchable archive of much of these early conversations online. Even the most careful privacy advocates were surprised to find their old conversations, freely given in a public forum, suddenly given a new lease of life. The argument against Google making the data available was hard to make: the messages had always been intended to be public, just not this public, not this permanent, and not this accessible. Even the forward-thinking pioneers of the Net had not envisaged this embarrassing return of public, but still embarrassing, data.

Can we expect to see similar future shocks for activities we now imagine as harmless? I think we can.

Two disparate projects might give you an idea of where the next retrospective leaks might take place. First, a small Google-sponsored endeavour at Stanford, called Cityblock. Cityblock was a research project with a simple aim - to photograph the shopfronts of as much of the continental US as they can. The hardware would be a high-resolution camera stuck on the side of a reasonably fast-moving truck. The researchers reckoned that they could capture almost all of the commercially active areas in the US within a few months.

The project began in late 2002, and all record of it has recently been removed from the Web, but I have a copy of the presentation that was given to Google in November of 2002. At that time Larry Page, one of the founders of Google, requested that the photographs be of sufficient resolution that you could "see the opening times of the stores" on the images.

There are some obvious - and fixable - privacy concerns with a project like this. What if you don't want your shop photographed? Simply ask for it to be removed.

On the other hand, the advantages of a project like this are obvious. Check out the styles in fashion shop windows. Print out a copy of the building you're about to visit, so you don't get lost. Such a large amount of data provides endless opportunities.

Google aren't the only ones interested in collecting such an album of photographs. The French Yellow Pages has already done the legwork for large swathes of the French roadways at http://photos.pagesjaunes.fr/.

Now in Cambridge, England, computer scientists are working on a system that can identify buildings in the background of a photograph and, using its own limited database of roadways, locate where the picture was taken.

As far as I know, the three research groups have never consulted, but it's obvious what might happen if they put the planned Stanford data with Cambridge's algorithms.

Now, while I'm speculating - don't think of the future. Think of the past. Think of all those snapshots people have put up online. Did anyone expect, when they uploaded those pictures, that one day someone might be able to find out where exactly you took that photo. Perhaps to uncover your home address? Or, who knows, with the help of another research project, to calculate using the direction of shadows in the background, exactly what place and date the photo was taken? Stalking you, as it were, through your past?

All of this is hypothetical, of course. But it goes to show that if you think that keeping your private life safe is simply a matter of keeping your own secrets, you may find that technology - and what tiny parts of your current life you leave open to inspection - may catch you out in the end.