Quite often the characters have a box on a string slung over their shoulder. What is this? Some sort of standardized first aid or fast response kit? Quite a few of the characters carry them on a regular basis and sometimes they are set dressing hanging from a coat rack.
There also was a law against carrying your gas mask at that time, I believe. (Just going on memory here, not any references.)
The "wardens," the civilians who patrolled to keep order and monitor these kinds of things would challenge you if you didn't have yours with you on the street, etc.
My fading memory tells me I saw that in a write up in the Museum of London where they were displaying gas masks from WW2.
The gas mask and filter HAD to be kept in a contained and protected environment. The rubber they were made from was a very serious war-time resource. Leaving one exposed not only increased the chance of possible contamination, many filters made then would degrade when exposed to air, reducing the effectiveness of the filter. Keeping the mask and filter in a container not only protected the mask from damage, it reduced the air-flow around the filter - which extended it's life(and possibly the owner's as well). Civilians were trained on how to properly put one on, and those who had civil defense positions were required to carry their masks at all times. If caught without their mask, they could loose their position, and face serious charges.
Even today, most modern filters used for extreme chemical hazards have seals on both intake and output ends of the filter. In some countries, removing a seal without reason can get a soldier in serious trouble during an inspection.
Gas wasn't used very much in WWII warfare. Both sides knew the other side had gas/chemical weapons, and possibly much worse. Because of this, both sides avoided the use of either weapon(mostly) against other troops.
Mustard gas was around but both sides didn't use it but there are always accidents in war when things and people are pushed to their physical limits.
In November 1943 the American Liberty ship SS Harvey transported 2,000 M47A1 mustard gas bombs containing 60-70 pounds of sulphur mustard each. It arrived at Bari, Italy, but the harbor was packed with ships to be unloaded. The captain was under orders not to speak although he wanted the bombs expedited.
On December 2, 1943, German aircraft attacked the harbor sinking 17 ships including the SS Harvey. Over 1,000 people were killed by the planes.
The Harvey burned and sank releasing a cloud of mustard gas on the city. 628 military men were hospitalized and 83 died. Civilian casualties are unknown. Most fled the city and took shelter where they could. Its believed the civilian casualties were higher.