I placed a gorgeous little pit bull puppy that I had taken in (abandoned at the clinic after parvo diagnosis & treatment) and found it a wonderful home with my younger brother's best friend. Someone did the same thing to that puppy. The neighbor, who confessed to the police, gave him antifreeze laced raw meat twice in a single day. By the time Sonny got Cowboy to us, there was nothing we could do. He was basically D.O.A. Sonny never got another dog.
Crap like this happens all the time. In Sonny's case, his whacko neighbor didn't want a "vicious blood lusting pit bull" living next door to her and she was "afraid for her children's life" so she did what "any loving mother would do to protect her kids". And I remember quote for quote that those were her words. Mind you this was a 4-month-old puppy. She actually got charged with animal cruelty, but Sonny never got anything but his new dog's ashes in an engraved box that he keeps in his bedroom.
I would be super observant and comb the yard like a hawk before letting her out. I would also take the initiative and talk to the immediate neighbors about it. AND the next time it happens, keep the bread, put it in a ziploc container, and call your vet to see if they can send it off for testing. If it has been contaminated with something intentionally, I'd be all over the local news media letting it be known.
I wish you the best with figuring this out!