That is somehow super creepy. What if the monkeys became some sorts of cryptids and are just out of sight/behind everyone. What if their master is the legendary three headed monkey. What if the monkeys are just behind the camera and thats why we can't see them...
What if we're the monkeys? The monkeys are listening, remember? They were in the audience in the theater in MI3. And monkeys are now associated with being behind someone, out of their view just like the fourth wall. When they look around to search for the three headed monkey there is nothing tho, because they are looking the wrong way... Would be funny and cool if they took all this as foreshadowing (aftershadowing?) and made a small plot out of it.