Detecting another object collision from another object script (2D)

I want to detect if a certain object collided ( or entered trigger ) with another one , but from a scripts that is not one of the two objects . for example I want to make the enemy walk to me when I enter a trigger object but without making a scripts for the trigger object to send a message to the enemy , I want the enemy script to detect if I entered the trigger of this object from his script .