The legend says: N6067U @ EDDF 06.12.2011
Boeing 747-8 Intercontinental N6067U in basic Lufthansa colors - this is the first landing of this type at Frankfurt.
It arrived as flight "BOE021" directly from Boeing Field KPAE/BFI and will perform final handling tests, in preparation of the delivery of the first Lufthansa 747-8Is in 2012. The aircraft remained indeed in FRA for tests from 6 to 9 Dec 2011.
Now, if the stop in BRU was unscheduled, the people spotting in FRA might not have known.
The other source I found is a forum http://www.flyertalk.com/forum/miles-mo ... ead-5.html showing a map from flightradar24 wity flight BOE021 (hex code A7E0E9) just before the landing in FRA. The plane does not seem to come from the direction of BRU, but from a point more North.

Whether this flight landed in BRU or not remains a big mystery...