The ESXi host client must be configured with an idle session timeout.
The ESXi host client is the UI served up by the host itself, outside of vCenter. It is accessed at https://<ESX FQDN>/ui. ESXi is not usually administered via this interface for long periods, and all users will be highly privileged. Implementing a mandatory session idle limit will ensure that orphaned, forgotten, or ignored sessions will be closed promptly.