We’ve attended a spate – well, three in a fortnight – of customers who’ve been locked out by an estate agent (“realtor” to any kind readers from North America). What happens is that soon-to-be-departing occupants leave for the day without the deadlock key. Either they were never given it by said estate agent when they moved in, or they’ve just got into the habit of not using it. Then the estate agent shows a prospective new occupant around and deadlocks the door, with they key they forgot to give you or that you forgot to take out with you.
Of course when you return and discover that you can’t get in, it’s probably past closing time and there is no reply from the agent’s office.
A typical deadlock key is heavier than a flat rim cylinder key (and moderately rare outside the UK), and one or two customers have said that that’s why they don’t put them in their pocket/purse.
So, if there’s a deadlock on your door and you don’t have a key for it, don’t assume that no-one has a key for it. And don’t let your door be the only one in your block or street that is closed only on the latch lock; get your pocket reinforced or a bigger handbag.