Yes, now I'm sure too. It was 100% visible to me today when we had a minor outage on that server. The Container wasn't reachable, and when I tried to log on to Plesk, I got the sidebar but noting else (the rest was loading infinietly). So when you log on as admin your sidebar is different, and it comes from Virtuozzo.
In virtual environments it seems to be a vz wrapper "before" Plesk.
The good news is that it's actually more useful than the skin since you can do something even if your container is down.
And as observed initially, Clients get the normal version so if plesk is down they neither get the sidebar.
Topic seems to be discussed, doesn't it?