"Ever gone away and not been sure if you remembered to lock it?"
1. What Roland6 said.
2. The door has a glass panel on either side. Therefore with a cylinder lock (AKA Yale* lock) latch it can never be locked at all. If the back of the door is accessible from outside then it can't be locked without a key or combination on the inside as well as the outside. A glass panel in or beside the door combined with a cylinder lock is a gift to B&E merchants.
Some of the comments in this thread reveal a worrying naivety about keys. Does nobody change the door lock when they move into new premises? A few years ago my daughter moved into a new house having received "all" the keys from the previous owner. As she was moving in a neighbour rolled up with another front door key that she'd had for some time that the previous owner had forgotten about. I'd have changed her lock anyway but it moved things on a bit.
* Yale don't just make cylinder locks. In fact my mortice lock was made by Yale.