Detect if user is logged in (system context service)

Hi all,

I have a service running in system security space and I need to check by a
registry key or environment variable if a user is logged-in to the
workstation.

The service MUST run in system space, but the environment variables named
"USERNAME" is defined in user space, thus not visible by the service.

Is there any indication in the registry (HKLM) that tells me if a user is
currently logged-in ... for example something that puts the username if a
user logs-in and removes it on closing the session.

Many thanks in advance,
Romain