Left Behind Item Alert

My father is constantly leaving his coat behind in places. What I need it a device, or pair of devices that will beep if separated. The only things I have found are either too bulky, or only beep when triggered. The problem is not to find the coat once lost, but to alert the user that the coat is being left behind. Ideally one half should fit in a wallet, and the other half should be sown into the coat.