From the Super Mario Wiki, the Mario encyclopedia
The Security Thwomp is a massive, stationary Thwomp that appears in Super Princess Peach. It is a wall-like security device guarding halls in Areas 8-3, 8-5, and 8-9 of Bowser's Villa. It has its eyes closed most of the time, but periodically opens them and scans the room at intervals, freezing the screen as it does so. If it detects Princess Peach, it will inhale her to send her to the start of the room. In order to avoid this, Peach must imitate the statues in the room, some of which require certain abilities with Perry. Like Beach Koopas, Blindfold Boos, and blue Podoboos, they do not appear in the in-game glossary.