Did the reports that the backyard gate was open indicate where the gate was located? Does it open to the front of the house, like on the side, or at the back of the yard?
I can't tell if the fence that runs from the right side of the house (as you face it) to the neighbor's garage is their fence or the neighbor's. There appears to be a gate, but no handle or latch on the outside. There are 2 fence panels that are cut shorter than the others, which may be low enough for an adult to reach over from the outside and reach a latch on the inside. There is also a large rock right near the gate. I cannot tell if it is meant to keep the gate pushed shut from the outside or not. Not sure if this is their fence or gate though.