It happened quite a few time in BRU, in 1982, 1995, 1999, 2000, 2001 and 2005. Either diamonds, gold or other valuables where stolen then. The biggest heist in Belgium however happened off airport in the diamont center of Antwerp where 100 million EUR (official number, unofficially much higher) mostly in diamonds were stolen in 2003.
It was reported that this heist had a value of 50 million EUR but apparently they came back to this number and are now saying that they don't know the value of the heist. Thus it can be much lower but also much higher. The official version is that 120 packages of diamonds were stolen during the heist.