It could be pornography, a bad drug habit, an extramarital relationship. Most of my work involves trade secrets, but oftentimes I'm retained by a spouse to look for hidden assets or contact by a paramour. The smaller ISPs are generally the worst in protecting your confidentiality. Say someone has been fired and they're contesting it. So I review their computer and find porn or excessive eBay shopping.
Employers can use that evidence to squelch the lawsuit.
In three quarters of cases I find porn on the machines, and attorneys use it however they can. That's their job. We're getting more and more calls to review the hard drives of employees who claim they were fired because of age, sex, or race discrimination. It's a fishing expedition, but it works pretty well for most of our clients. That's a violation of our company policy, so you could've lost your job anyway.