They seemed not to wear masks at times when the weather was cold and bright, if they were outside, or inside the tank when it was closed. They wore them inside the tank when there was danger close by, as in escaping from Copenhagen, and I presume wearing masks in the shop was only sensible, since, while Lalli had checked the place, a small vermin beast might wander in from somewhere else, or they might unwittingly pass through a shady area where a troll had recently breathed.