The definition of surveillance is close watch over something or someone.
A private investigator can be used in different situations when it comes to surveillance. A corporation can use a PI (private investigator) if they believe someone claimed workmen's compensation under false allegations or for an internal investigation into a company. A private member of society is able to use a private investigator in divorce proceedings or personal situations.
Have you thought about speaking to a private investigator to see if they can be of help in your situation?