Lucy has been able to unlock the deadbolt and the doorknob lock for a few months now, but it hasn't been a problem until recently. She has crossed the street once without one of us with her, and she has repeatedly opened the door and stepped out onto the porch before I could stop her. We have told her about cars hitting her, people taking her away, but chose not to mention the fact that if she unlocks the door while I am not aware, someone might be able to come into the house and do bad things.
Since she stopped napping a year and a half ago (I know I could "make" her... trust me, it's NOT worth the 2 hour battle for a 90 minute nap. If you have a kid like this, you might sympathize. If not, you will have to take my word for it, I am tougher than a preschooler, but I have better things to do with my time than force her to sleep). She wakes me up in the morning around 7:00 asking for breakfast, and she is up and doing silly, busy things until she falls asleep around 8:30 PM (extended because of the later daylight). In order to do anything, literally - shower, throw in a load of laundry, change Henry's diaper, anything - that might require me not to be staring at her, that means she is unattended for short periods of time throughout the day.
She decided to use a couple of those short periods of time to go outside without one of us too many times. This morning she went for it, and when I pried her hands off the doorknob, she fell face down to the floor, requiring me to drag her out of the way, in order to close the door and lock it again. I immediately went to our cabinet in the mudroom, found another Safety 1st doorknob cover, and clicked it in place over the doorknob. Lucy swatted at it with dramatized futility, and as it loosely spun around the doorknob... my blood pressure dropped 3 points. That battle is over.