Estate Agents' Reading Skills

After wondering at the Labour Party's members' reading skills, I now worry about the same skills with estate agents. I just had one of them trying to open my door in order to show a flat to a customer, when in fact he wanted the flat next door. Of course his keys didn't work. Now, my door has a big letter A on it, while the next door has a big letter B. This is meant as a clue that my flat is flat A, while flat B is next door. But apparently, that was way too complicated for him.