I am not an accountant, but I believe that such unused balance can not be treated by the company as an asset, and instead as liability. And I believe there are laws in every state where such money eventually should be returned to you or to the state's "unclaimed property", if they can't contact you. Search for "unclaimed property <state>" and see if you can locate anything there.

On the other hand, I have no idea how that is enforced. So it is quite possible that liability stays on books indefinitely. Or worse - not recorded as liability in the first place..