Oyster cards, the "smart" little blue thing in London commuters' wallets that enable them to travel at will around the capital, have another, unexpected function. They could also be a one-way ticket to the divorce courts.
For the cards, introduced by the London Mayor, Ken Livingstone, in 2003, don't only let you on to public transport - they also record your every journey. And, private detectives and lawyers report, that is information the suspicious are accessing to track their partner's movements.
And they are increasingly finding that the night she said she was working late in the City, she was actually hopping off the Northern line at Morden, staying there for some hours, before returning to town at 11.17pm. Or when he said he was playing football on Hackney Marshes his informative little Oyster card reveals that he was really catching a bus in Parsons Green.
The electronic lipstick-on-the-collar is revealed to anyone - the holder or their partner - who takes the card to a machine on the Underground or keys in its serial number on a website to get a read-out of every journey taken in the past 10 weeks.
One private investigator said: "Oyster cards won't tell you that the bloke's been cheating on his wife, but it will show if he's been in one part of town when he's supposed to be somewhere else. It is an easy thing to confront your partner with. It doesn't look like you've been snooping around too much."
The use of the cards is the latest weapon in the growing high-tech arsenal of tools used by suspicious partners. A telltale sign such as lipstick on the collar has been replaced by technological snooping such as placing a tracking device on a mobile phone.
Several internet sites now offer mobile phone tracking as a service to worried parents keen to know the whereabouts of their child. But private investigators said spouses who suspect that their partner is cheating are increasingly using mobile phone tracking sites.
Although an SMS message is sent to the mobile that is being tracked confirming the process, if this is deleted before the "target" sees it he or she has no way of knowing they are being followed.
Peter Heims, spokesman for the Association of British Investigators, said technology was transforming the private detective industry.
"Trackers are used on husbands all the time," he said. "You can get one with a magnetic bottom which you stick to the underside of the car. It will track the vehicle via satellite and is accurate to within 10 feet. A lot of companies now use this to check where their travelling salesmen have been."
From software that records every single tap on your lover's keyboard to DIY lie-detector kits, the market for catching a cheating spouse is now bigger than ever.
A company called OverSpy will let you monitor everything your partner does on computer by sending you email reports of the websites visited and emails sent.
Another firm has software that allows someone to retrieve secretly text messages deleted from a partner's phone. All that's needed is the SIM card.
A quick search reveals more than 14,000 websites that will help uncover a partner's infidelities, whether it is opening letters secretly or tapping a partner's mobile answerphone.
Proving adultery is no longer necessary for a court to grant a divorce. But the growth in technology in this area has enabled partners to check their suspicions in a way that was never possible before.
Divorce lawyers said they were sceptical that Oyster cards would be used in divorce proceedings, but accepted that it could lead more people to realise their relationship was over.
Lisa Fabian Lustigman, a family lawyer at city firm Withers LLP, said: "I would never instruct a private investigator to try to track down someone's Oyster card records to prove adultery. I don't think it would be overwhelmingly helpful."
But it has already happened in Hong Kong, where a similar scheme was introduced eight years ago. Suspicious husbands and wives obtained print-outs of their spouse's travel card to use as evidence in divorce proceedings. In the former British colony, the "smart" cards are used for shopping as well.
If London follows Hong Kong's example, Oyster cards will soon become even more useful for the distrusting partner.
Transport bosses in London hope to expand the concept here, allowing card holders to pay for their shopping in nearly 4,000 shops with their travel card. The records of where a person has shopped, as well as where they have travelled, will then be stored on the card.