Helped our lad move house at the weekend, and upon first time arrival at the new place with his friend (lad stayed behind to carry on emptying his flat) there was an issue with the front door key. Or so it seemed.
Closer inspection (and a nagging feeling in my mind) revealed that the lock mechanism on the door has a latch such that the door can be closed and secured *without* the need for a key. Once latched, you need the key to move the latch mechanism and open the door. I suspect there's a little bit of tweaking needed - job for the landlord.
This is *exactly* the design of lock I have tried to describe several times since 2008 when our front door was fitted to a succession of blank looks and completely lack of understanding when I grumbled that I had lost something with the "new" lock.
Oh well ....