When an unauthorized person enters the target area while the effect is running, a caterwauling noise will be set off.
References from the canon
- Hogsmeade was under a Caterwauling Charm that would go off if anyone moved around outside while curfew was in effect (DH28).
Magic Type Charms
"caterwaul" Eng. to make a screaming noise like that of a cat during mating season