The first option is simpler. Automatically start a battle with the creatures on the computer's turn if your hero is close enough to them at that time. Plus, display a message, for example, "You have been spotted and attacked!" Something like that.
The second option is a bit more complex. Create another player and give them heroes that look like neutral monsters on the map; the hero is not visible in battle, only the army. However, you'll also have to come up with something to prevent them from taking turns in battle; I don't know how to do that yet. But otherwise... it's a regular hero. They are inactive when no one is nearby. Someone is nearby => they activate and try to catch up (their movement range is smaller than that of regular heroes). The opponent ran away? They deactivate again. That's the idea.