Ridgeback- do you have a role to go back to or were you recruited externally to do this acting role?
thebrainy123- are you familiar with this firm's HR policies and /or agreements with the trades union about advertising vacancies?
It's common in my experience for someone to 'act up' temporarily (maybe via a temporary promotion or sometimes just as a development opportunity) when the length of absence isn't known, and then when it is decided the absentee won't return the job is often advertised if there is a union agreement that this will happen.
But I agree with the brainy123 that if they really want you they'd drop a hint or find a way of appointing you.