Insurance companies may hire a independent private investigator to perform surveillance on you. The purpose of surveillance is catch you doing something outside your restrictions or even to see what you are doing during a typical day. Often times surveillance will be done in cases when the employee has been off of work for lengthy periods of time, there are signs that the employee is magnifying their symptoms, or there is a question about the legitimacy of the injury.
In some cases, the investigator may be outside your home in their vehicle, they could be watching you from afar while you go the store, or they may even follow you around. Obviously, this can be intrusive on your privacy and even frightening to have someone follow every move you make.
Now, I am not trying to scare you nor am I telling you that you should be paranoid. However, I would say that you remain cognizant of what you are doing. If a doctor has given you restrictions of no lifting more than 5 pounds, you should probably avoid helping your brother move his couch up six flights of stairs even if you feel great that day.
Always keep in mind where you are and what you are doing even if you feel you are having a good day physically.