Cause the True Depth camera is very wide angle and it is seeing you while you watch your brother open your iPhone. This is an old joke that people do with the iPhone. Want a real test?
Go into Settings > Face ID and Passcode, make sure an alternate Face ID has not been added (erase it there is one and you didn't add it) and erase and do a new Face ID for you. Then set a new passcode (six digit) that your brother doesn't know. Restart your iPhone. Hand the iPhone to your brother screen side facing down and tell him to go into another room, unlock your iPhone, open Notes, and type and save a Note that says "I can open your iPhone".