Probably mostly my fault. Since I run around without a top and doors so frequently I tend to forget to lock my doors when they are actually on.
Anyways I went out this morning to find my passenger door open. No one has used that door in the past week or so, so I know it wasn't a "forgot to close".
Luckily, nothing was missing (except $0.32), but this is also my fault, as I am so used to, again, running around without a top or doors that I make habit of leaving nothing of value in the car.