Though I'm not sure that the police officer should be fired since I don't see how a police officer who loses his gun necessarily means that he wouldn't be an effective cop in the future but Kokopellian raises a good point .. that if the gun was later shown to be used to kill somebody, Fairfax County might be liable. In light of this, it would be absolutely essential that Fairfax takes action to prevent something like this from happening again.
This is not the first time I have heard of law enforcement losing guns (though this is the first time I've heard of an officer losing a weapon whereas they are typically stolen). I wonder if officers are required to pay for the replacement cost of their firearms if they are stolen/lost? It would certainly encourage officers to take better care in storing their weapons.