Mirrors can be more like vessels than portals. But I do know that some witches use mirrors to create portals for spirits to enter/exit. Mirror magick is very old, it has been used as a way to scry since ancient times.
When I was younger I would use a mirror to see inside the doorways of dark rooms and could see the details of spirits more easily than with my eyes alone. I never learned how to do this from a book, I just realized from a young age that the reflection worked. Don't try this if you get easily freaked out!
If you see something in a mirror that shouldn't be there, it is time to clear, cleanse, and protect that mirror and banish anything inside. Especially if your reflection changes in a distorted or sinister way. Images behind you are more likely to be outside sources. But if they form in front or over your reflection, odds are, it's in the mirror.