How can I ask a SM to produce documentation on when they returned from deployment?

When a servicemember is returning from deployment, is there any document given to SMs that states when someone is returning? I used to be in the Army but don’t have a ton of experience with deployments. I work HR now and we have an employee that I personally suspect is taking advantage of USERRA and thinking we aren’t any wiser. We can’t reasonably hold his position much longer if he doesn’t have a reason he can’t show back up to work but Imm looking for what documentation we can request from the SM in order to return to work. Right now his absence is causing increased workload on others in the organization and of course we respect it if he’s literally on orders out of the country, but we’ve heard from another employee that he returned to the US a while ago and has not notified us that he’s planning to return to his job. His manager likes the guy well enough, but we need to get fully staffed again in his department and cannot just hold his position indefinitely if he has returned from deployment.