Discuss mental wellbeing
Managers often shy away from asking an employee with a mental health condition how they are doing for fear of "opening a can of worms" or getting an answer that they do not know what to do with. You probably would not think twice about asking an employee returning to work after breaking a leg how they are getting on.
Anyone can experience a mental health problem, so being able to talk about it in the workplace is important. The more conversations you can have with employees about their mental wellbeing -- not just those returning to work -- the more likely you are to identify issues and prevent further absence. You could make it a regular part of your one-to-one catch-ups to ask employees how they are coping with work and how they are doing more generally. It will not take long for this to become the norm and part of the culture of your team.