Medds interfaces the chemical concentration of the vapour from explosives with the nasal sensitivity of specially trained dogs, while the Fido system is based on such vapour interacting with an amplifying fluorescent polymer.
Part of the large garage behind the Alexandra Hotel was converted to a First Aid Post, 'Medds' Garage and the Windsor Garage were used to store around 20,000 gallons of fuel, which was only to be used in the event of an invasion.