Well, first of all, HIV isn't a disease that can "lay dormant" like some websites suggest. Diseases that can do that (like herpes) hide in the nerve ganglia of the body and only manifest themselves at certain times. HIV does not do this.
The reason that HIV may appear to lay dormant is because the disease has not yet progressed far enough to show symptoms. Thus, a person may not know that they have HIV for months or even years.
This is the reason that some people may have a negative HIV test result when they are actually positive. It can take some time for the immune system to produce enough antibodies for the antibody test to detect, and this time period can vary from person to person. This time period is commonly referred to as the "window period." Most people will develop detectable antibodies within 2 to 8 weeks (the average is 25 days). Even so, there is a chance that some individuals will take longer to develop detectable antibodies. Therefore, if the initial negative HIV test was conducted within the first 3 months after possible exposure, repeat testing should be considered >3 months after the exposure occurred to account for the possibility of a false-negative result. Ninety-seven percent of persons will develop antibodies in the first 3 months following the time of their infection. In very rare cases, it can take up to 6 months to develop antibodies to HIV.
Another type of test is an RNA test, which detects the HIV virus directly. The time between HIV infection and RNA detection is 9-11 days. These tests aren't routinely used though and are quite expensive.
So, in answer to your question, HIV doesn't mimic normal cells or hide in the body. Once the body has produced enough antibodies to the infection, you will test positive. If you have had a series of negative tests, you are negative.
Does this help?