What should IT do with the computers of employees on long-term leave?
Usually switch them off or take them in, and make sure their accounts cannot be misused in the meantime. Agree a simple routine with HR for leave of more than a few weeks, such as parental or sick leave or a sabbatical: the laptop is stored or kept switched off, desktop machines are shut down, and the account is suspended or left active only with multi-factor authentication, depending on whether the person still needs remote access. When the employee returns, update the machine before it rejoins the network. Treat this as a security step, not as a judgement on the person, and say so in the policy. If a machine must stay on, for example for a shared task, give it an owner. NetSenX alerts on every plan if such a device starts beaconing, scanning or uploading while it should be quiet.
General information, not legal advice. National laws can add to the EU texts quoted here, for example deadlines, forms or reporting channels: check your own country's rules.