No surprise, but Lincoln likes balls. He also REALLY likes being outside. He is an ambitious little guy and has decided there is no reason he should be inside if he doesn't want to be - so he's figured out how to unlock the front door.
The first time one of the kids told me he had left/unlocked the door I dismissed them as slightly crazy. But then I saw him in action myself - yikes! I've told him that it's not okay to leave the house on his own but he is just a tad on the strong willed side. I'm not sure that he took my admonishment to heart.
So... door alarms or a better deadbolt? Off to research!