Multiple users, xref notification

I know this must have been asked before...sorry for asking again..

We have multiple users using a single base file as an xref. One user is
actually working in the base file and makes changes periodically.
Normally, when the base drawing is saved, there is a small yellow
notification bubble in the corner of the screen telling users who have
a drawing open accessing that xref that the drawing has been updated.

In the case where a destination drawing is NOT open and is later
opened, the xref'ed base drawing is automatically updated as part of
the drawing loading, without notification.

What I am looking for is a way for the second user to know that a
change has been made since their last opening of the drawing. The
simplest answer is "User 1 opens his mouth and tells User 2 that the
drawing was updated and where...etc.". Is there an AutoCAD method, in
the stock program, in a thrid party free download or a 3rd party
release that solves this?

Just curious....right now, we are on the 'yell across the room' method
and it gets a little bothersome. Another method I thought of was using
Instant Messenger between users so changes can be announced outside of
Autocad and a brief list can be "posted" to each user

Help!

Thanks!