"Don't leave keys in doors, but keep them somewhere local to it. If people leave keys in windows locks or doors [a burglar] could get to them easily," he said. ... "This way, if there's a fire, you can drop key out of the window to fire brigade who can come in and rescue you.

Is it safe to leave a key in a lock?

Leaving the keys in the back of the door is a bad idea for the same reasons and is particularly risky if the front door has a glass panel in or near it. The glass can easily be broken to steal the keys from the lock allowing intruders to gain access into your property.

Can you pick a lock if a key is in the other side?

Yes, you can, providing that your door has been furnished with a frictioned cylinder lock or one that has an emergency function. These two types are designed to be unlocked with a key even if there's another key inside on the other side of the door. Their mechanisms are created for this exact purpose.