Several things about mirrors. Almost all monitor cages have mirrors. That is, the glass functions as a mirror. If the lites in the cage is on, and outside the cage is off, the glass becomes a mirror, on the inside of the cage. And of course the monitor will see the reflection, of whats inside the cage. So I get the feeling captive monitors learn what a reflection is. I also have seen many instances that monitors know the real direction, a reflection comes from.
Now to add to this and this is actually the most important. Most people have no or little understanding of monitors, or heck animals. You know, I know, that with monitors, sight is only part of their senses. Sight is about awareness, if a monitor sees something, its becomes aware of it. Then it needs to smell it. As smell confirms and identifies the object. If sight and smell does not identify the object, the touch is next. Monitors touch with their claws, but more importantly their nose. When unsure of something, they poke it with their nose.
Monitors like us and other animals, have more then one sense, and they use them. They most likely have senses we do not have. Like some sort of electrochemical or chemical organs on their face and lips.
You have mertens, yes? They have very sensitive organs on their face. They are very sensitive to metal. It blows their minds.
I believe most if not all monitors have this to a certain degree. Ever wonder why the dang things are so careful about keeping their face clean? Cheers