We leave stuff outside most of the time, depending on where we are. Chairs, bikes, grill... If there is a lot of activity with people or kids running through or passing, I do not think it is a good idea to place temptation in the way by leaving things like Iphones, cell phones, cameras laying around. Some people do not have enough respect for others to leave things alone, or have never been taught that they can't have everything they want.
But generally, If you are in a place where you have to worry about it, you are in the wrong place.