There are some people are good at hiding at, something called talent I guess. But, I do believe that hiding something would not last forever. If you don't caught yet of what you doing, maybe you are good at hiding for a moment, but watch out because it won't last long. In time, you will get caught not unless you stop or the person doing it will stop and won't do it again, then no one can caught him from doing such thing. People can be easily detect or knows something is going on, try to observed their eyes, their reactions, their facial reactions, and how they responds on to something you ask. Only lie detector test can proved if the person is telling the truth or not. But try to physically observed the person and study what he is been doing too lately.