[MargaretR]

They have been installed in the ground floor flats below mine.
Those flats are classed as 'sheltered accommodation'.
The tenants have been told that they are there to avoid the need for breaking the door down in the event of medical emergency.
I suppose they do not want others to be able to remove the key in case it doesn't get put back.
There is nothing to prevent your mum from having an extra key cut for the carer if she wishes.