When therianthropes are taken into custody by the governemnt, they are injected with a microchip. This chip allows them to be tracked and also identifies them. At the same time, a capsule of poison is injected. The chip contains a GPS unit which has the location of the Sanctuary walls hard-wired in. (GPS was used because ti was the only way to make sure flying theris were as trapped as everyone else.)
Crossing the walls triggers a fatal release of the poison. The Department of Therianthrope Control also has hand-held controller for these chips that they give out to their employees who interact with theris personally. All this stays inside the theri when they transform.
After the WallEdit
When the Wall was built there was no change made in the chips. The anti-theri government faction allowed living conditions to be improved for the ‘tame’ theris, but they didn’t want to risk them escaping.
At Separation, the Pack left Sanctuary the same way they entered it: a member of DTC temporarily shut down their tags while they crossed Sanctuary's boundaries. The poison capsules were not removed. The GPS tracking was left on. Now any attempt to cross back into Sanctuary would kill them.