I would think that the listening is done outside locked rooms, rather than unlocked apart.ments. Nevertheless it is a pretty old-fashioned method imo. In the 1950's when Butlins did it there was no thought of predatory people stealing children, and children were less protected also; they played out, and i had a door key at 5 years of age. I'm amazed too that parents do this today, when they wouldn't dream of letting their children walk to school or play out all day.
Weren`t the chalets also of the old-fashioned "sleeping quarters" type, in rows........as used in the Hi-de- Hi T.V. series?
Bedrooms, in effect.........and locked.
The shower / bathroom blocks were external and communal...........as were the restaurant facilities.
Didn`t self-catering apartments and caravans come later , with listening services considered unsuitable for this type of holiday home?
In the same way.........hotel listening services through locked bedroom doors are not to be compared with unlocked apartments opening onto a road or car-park ............and which are not thought suitable for "listening services" by the holiday company in the first place?