If someone on house arrest needs to go to the store, they are permitted to do so. The process involves listing places where the person would normally need to go, such as to the store to get groceries. The ankle bracelet itself is a GPS unit, and as long as it leads to the store, then that's fine. Otherwise, when the person goes outside of the allowed areas, then it logs it and the cops will investigate.
Ankle bracelets don't necessarily "go off" as they do log information.