I believe that you, or rather the PE under whose supervision you work, have an ethical obligation to formally report this situation to the owner or manager of the building. I would do it in writing, and send it by registered mail with proof of delivery sent back to you.
You came across something dangerous, whether or not it was part of the task you were being paid to do. You can't disregard the possibility that this situation can result in the injury or death of a tenant in this building. Put the duty of dealing with the issue into the hands of the owner or manager.
Feel free to PM me, if you want to discuss further.