Just spoken to Nairobi... The emergency services are responding to a reported 747 200 problem. HOWEVER, details are very sketchy, this is not confirmed by any means, and there is also the large possibility that this is a 'simulation' to check emergency services readiness. Will get back to you if I find out more. Please post more if you get.
Can now confirm that it a simulation. The airport runs simulations every now and then to see how the whole place reacts.