Technology correspondent, BBC News
Privacy campaigners are convinced that big companies, from Google to Tesco, know too much about us - and are not careful enough with our data. We asked a young Londoner to find out what three big organisations knew about him.
Dan Senior used to be a lawyer but gave up the legal world to go travelling.
Right now, he lives an easy-going life with his girlfriend in their flat in London's Bethnal Green. He spends a lot of time online playing poker and using Google for his searches.
He also holds a Tesco Clubcard - and he travels on the London Underground quite a bit, using an Oyster card.
That means Tesco, Google and TFL, the Oyster card operators, know quite a lot about where Dan goes, what he buys and where his net surfing takes him.
After popping out to Tesco for some milk and a bottle of wine, Dan phoned the Clubcard helpline. He wanted to find out what the firm knew about him, how long it kept any data - and whether it was shared with anyone else.
"We only keep your name and address," said the man at the call centre. "Nothing about your shopping."
That seemed surprising - and Tesco later confirmed it was inaccurate. While call centre staff don't have access to your data, details of all purchases on Clubcard are stored for up to two years.
A company spokesman said "No personal data is ever transferred or sold to any company outside Tesco."
Supermarkets track what their customers buy
Tesco acknowledged that data could be used to work out whether, for instance, the inhabitants of a town objecting to a new store were shopping at another Tesco some miles away but promised their individual names would not be identified.
Dan's call to TFL revealed that details of every journey he made were kept for eight weeks and some of the information was used by the organisation's marketing division to give passengers travel information. It also must release information in response to court orders.
Then we took a look at Dan's computer. He had signed up to a Google account which meant that he could see his web history - information that would also be stored by the search company.
Unsurprisingly, there were plenty of references to pizza and poker. There was also a link to an online dating site, a surprise and embarrassment to Dan, who insists he had not visited the site.
It was almost certainly a pop-up which he'd clicked on inadvertently. So what was happening to this wealth of data - some of it inaccurate?
It was off to Google's shiny London offices to find out what they knew.
There, Dan got the chance to put his questions via video-link to Peter Fleischer, Google's Global Privacy Counsel, who is based in Paris.
Details of Tube journeys are kept for eight weeks
He told Dan the company used the information for his benefit: "We keep the logs to refine our search business, to maintain security and protect people like yourself from spam."
Mr Fleischer said Google had recently changed its policy so that all the information was made anonymous after 18 months - and he promised: "We will never transfer any information to outsiders without your specific consent."
But what about that link in Dan's web history to a dating site. "Maybe someone borrowed your computer to make that search...", he said. "If there is something there you didn't want to have in your history you can go and delete it "
Outside Dan told me that the day on the trail of his data had been an eye-opener: "I'm fairly happy that my information isn't going off in all sorts of directions.
"But given the way the search engine business in particular is growing and growing ,I'm going to keep a close eye on it to see that doesn't happen in the future."