A common PI trick to ferret out cheating spouses involves doing searches for credit cards in the name of one of the married people and a person not his or her spouse;
apparently, many men who keep mistresses (and some women who keep lovers as well) will get a credit card in the name of the married man and the mistress, rather than the married man and wife, and use that credit card for expenses related to the affair, so as to keep the financial information away from the spouse.
I don't know about America, but here in the UK (and probably throughout Europe) credit cards are issued with only one name on. Maybe I've read it wrong and it means "a credit card account with two different names and two separate cards"?
Another way to do it is to trace movements of people using the credit card. Hotel reservations using a credit card when the spouse believes that the person is late at work or away on business or whatever, people checking into a hotel reserved in someone else's name using someone else's credit card, that sort of thing.
This all seems like a lot of palaver for some extra nookie. Why doesn't the straying husband (say) go on a trip to somewhere he legitimately goes on business or recreation, and get the lover to meet him there? Then he can show genuine bills for legitimate hotel when he gets back, if necessary.
I suppose that is a form of social networking.
