The Replicator Agent will process data from whichever ServiceNow instance sends it data by default. This can be both helpful and a tricky thing if you are accidentally sharing data to the production Agent from a subproduction ServiceNow instance. In later version steps have been taken to try and prevent this further. But one of the best options still is to take advantage of the instance filtering option on the Agent (https://wiki.perspectium.com/doku.php?id=replicator_agent_instance_filtering).
While you will still likely want to make sure other instances are sending to your Agent when you don’t expect it. Having this in place provides a good layer of security in case messages to make it through.