It's always part of my training that they are not to accept treats or food from anyone unless I give a command allowing it, my corsos have been the easiest to do with this since they seem to have an inherent distrust of strangers. Two quick funny stories for you, my neighbor bought a box of milkbones because he wanted to give my dogs some, he asked me I said it was ok, he came over held them out they sniffed the air and just look at him, he didn't get it, I told them it was ok they took them and ate them, this happened several times, one day I was out and my neighbor called me over he tells me that the day before the dogs were out but I must have run in the house, he tried to give them treats but they would come to him, he then tried to lean over the fence to get closer and they started to growl at him so he stopped there and just threw the milkbones into the yard, my response, I was wondering why I found two milkbones in the grass that day they never touched them and I had thrown them out. Then we were at the vet one day, the vet wanted to give my male a treat to distract him while she did something, he wouldn't take it, I said nothing as she said she was going to get a meat treat no dog resists, she came in with it and as she was trying to give it to him he just kept turning his head away, so I told him it was ok and he gobbled it up, the vet couldn't believe it I think he was 6 months old at the time, I let her try the same thing with my female and she got the same response. I started doing it less for fear of my dogs being poisoned and more to stop people from trying to feed them garbage, I just like to know anything going into my dogs stomach