I never thought of a mirror, but there was a time when this was such a problem for me that I put a sign up at my doorway. It said, "Warning, contents under pressure. Please knock before entering or speaking. Thanks."
This worked well to remind people, most of whom had seen me jump at least once already. Sometimes, I'd just hear someone laughing when they read the sign and I was warned that way.
And it's not abnormal at all. It's quite common actually.
If you understand everything, some things are just as they are. If you understand nothing, things are still just as they are.