Messenger Plus! is correct when saying the status is Idle. Because MSM Messenger use the same status (in this case Away) for both Away and Idle in MSN Messenger it shows up as being Away no matter the status.
As Stigmata posted there is a patch located
here that will alter MSN Messenger to show the Idle when the contact is in fact idle.
Which window do you mean when you right click and go Messenger Plus! Extras > Contact Information? If so I will let Patchou know about this if it is the case when I get home.