This happened a few years ago...my son and his cousin both 5 were playing in my parent's garden. My parents have a big garden, it's about 200 foot long but it's not enormous and it's surrounded by a hedge/fence all around. Anyway, they were playing outside alone which I thought was completely fine.
Anyway, my son came in and said 'Is someone going to steal me?' and I said 'of course not! Who said that to you?' A family member said, 'I did, because they were playing in the garden on their own, which I'm not happy about'.
I said I was fine with my son playing outside on his own but I had to keep reassuring him that no one would steal him. Now of course I had previously had a chat with him about going off with strangers etc. but had not felt it appropriate to worry him about abduction especially in an enclosed garden!