I have witnessed this problem too affecting dogs in our homestead. Your case has to do with the business aspect of the crime but in my area it has to do with vendetta and plain sadism as well as the familiar element of thievery. Over the years, we have lost a series of dogs through poisoning. In these cases, the common denominator to bait the dogs is pieces of meat. These outrages are usually carried out by neighbours or malicious passersby. Dogs which have been kept in homesteads as sentries are at an increased risk since they pose a threat to thieves and might also harass pesky neighbours. Some people go around this problem by keeping larger numbers of dogs if they can afford to maintain them or train them to detect poisonous compounds laced with meat.