My first reaction is it depends what you mean by "operating a business". If they are manufacturing items that would be considered a Factory occupancy, or selling items such that it would become a Mercantile occupancy, then yes, you have a change of use, and needs to be addressed as such. If the activities they are doing is consistent with a warehouse and storage activities, it would not be a change of use.
I would suggest the first course of action is to talk to the property owner and look at the lease agreements - often, the lease agreements for these types of facilities prohibit a lot of activities that may be happening. Allow the property owner to enforce terms of the lease agreement, if applicable, before you start enforcing major actions. Good luck!
