What is Ainsley wearing on her foot?

This a an electronic tag, that the Police use to track someones (usually an offender) location and ensure they are where they say they are at all times.

There are a number of different version available; some (more common these days) witha GPS location chip built in, and others that use a proximity chip to a sensor that is typically installed in the wearers dwelling, to ensure they are home (and in range of it) by a curfew.

They are typically very common for convicts on parole.