Can I suggest two possibilities, firstly it could be an amber beacon if the machine is likely to respond to any nearby airport incidents. More likely however, is its a red beacon to indicate at an incident that its the command point for the job
Added by Matt Ireland on 15 July 2010.
Just spoke to one of the "firies" at the station...very helpful chap. The "extra" light on the roof is simply a multi-directional auxiliary spot light which is controlled from within the cabin. It is just an extra light for the guys to use at night-time etc...hope that helps.
Added by Luc Galloway on 15 July 2010.