Regardless of the type of business you operate, visits from bosses, contractors, and family members will find themselves on site at some point. It is important to inform all staff of policies with regards to visitors so that there are no surprises. But, how would you handle it if you found out an unaccounted for visitor is there, that did not follow the visitor management process? Or if a visitor (invited or not) started to cause trouble?
If you have security on location, the first step is to give them a call so they are ready to come assist if the discussion with the visitor does not go well. If the visitor is on the premises to meet with an employee, then they should also be informed very discreetly. At that point, you can politely approach the visitor and ask them to leave if they are being disrupted. Or, at least ensure that they sign in appropriately so that visitors on location can be tracked.
It is important to address the issue quietly and respectfully, as you never know who you are dealing with or what their reaction might be.