If it could happen at the toilets in Parliment it could have happened anywhere. The officer didn't make a conscious decision based on their location. It shows a lack of due care.
It concerns me when I see people dismissing the seriousness of a matter because nothing terrible happened.
Is there a downside to it being brought to the attention of all armed officers to take due care of their weapons? We get refresher courses on all sorts of things at work, even if there hasn't been an incident specifically, because vigilance and prevention sure beats the alternatives.
The Police themselves prosecute people all the time for lack of due care... I don't see why they should apply a lesser standard to their own officers.
Plus of course the delicious irony of the Police themselves making a firearm available inside the cordon established to protect our MPs. One does wonder how often it happens, but the gun is recovered without it hitting the media. Perhaps it's worthwhile for terrorists to periodically scour the Parliamentary toilets for weapons?
I'd be inclined to agree that the officer should be punished. People are punished for their mistakes all the time!