You probably know that credit card companies have been scrutinizing every charge on your account in recent years, searching for purchases that thieves may have made. Turns out, though, that some of the companies have been suspicious of you, too. In recent months, American Express has gone far beyond simply checking your credit score and making sure you pay on time. The company has been looking at home prices in your area, the type of mortgage lender you're using and whether small-business card customers work in an industry under siege. It has also been looking at how you spend your...