The flag was initially spotted at a NATO gathering in Prague, where expansion of NATO was discussed. It was shown on BBC World (and probably most other TV channels) as President Bush addressed the NATO Prague conference attendants (image captured here). Behind him were the flags of NATO member states, plus the NATO flag itself and this flag that I have never seen before. It is the same shade as the NATO flag with in the centre a purely-white (i.e. no shades) logo resembling the silhouette of the NATO star with eight 'rays of light', two in between each of the star points.
I suggest a tentative identification as the flag of Euro-Atlantic Partnership Council which gathered in Prague after the NATO conference to discuss NATO expansion.
According to Lord Robertson's speech opening the December 2001 EAPC meeting and introducing the new flag the flag was designed by Janis Krievs from Latvia as part of an international competition.