A Groningen University related page shows the LIBER flag, flown at the 34th Annual Conference at Groningen, Caption: "Proudly, the LIBER flag flips in the wind in front of the Academy Building..." We see a blue flag bearing twice the acronym LIBER, in white, arranged in a circle.
Homepage of organization
"The Ligue des Bibliothèques Européennes de Recherche (LIBER) / Association of European Research Libraries is the principal association of the major research libraries of Europe. It was founded in 1971 under the auspices of the Council of Europe. Its current membership includes almost 400 research libraries in more than forty countries. LIBER membership is not restricted to the area of the European Union, and membership of research libraries outside the European Union is widely encouraged. LIBER actively promotes co-operation with all library-related organizations."
The words on the flag drawing (at the top of this page) are separated by two white five-pointed (upright) stars. Meetings in Uppsala 2006 and Warsaw 2007 did not show stars, but Istanbul 2008 did and so does Toulouse 2009 (forthcoming). What is lacking so far (I have not searched nearly enough, of course) is photographic evidence for a real flag with the additional stars.