So are you sure you wouldn't have been able to open the front door from the outside, even when it was not locked from the inside?
I haven't tried it, so I cant know. However we had a double locking device on our front door and I cant get in without a key even when it is only singly locked.
Since then i have visited London and found that estate agents have a way of getting in to single locked apartments using a credit card. But not some locks.
The key to 5A was a very odd affair, rather likie a twig of a Christmas tree with bumps at all angles along the operative stem. I expect you will have seen the key. I am very doubtful that a credit card could have opened a lock operated by such a key. But it would be a simple matter to home kit cast a copy for one off use .
The keys did go missing for a while according to reports.
Almost cetrtainly entrance was gained via the very dark and deeply recessed front door. No-one passed it so it was completely hidden and private.