My friend is diabetic. She has a mutt (Chow/Border Collie mix) who I htink can sense when her sugar is dropping. Her diabetes is not well-controlled despite her an her endocrinologist's best efforts, and seems too drop a lot in the middle of the night. It never seems to fail that she will tell me that the dog was all of a sudden fussing with her at 2 or 3 AM, waking her up, and when she wakes up she feels like her sugar is low and when she checks it, it always is. The dog doesn't do this every night, so it's not a habit of the dog, if you know what I mean.
This dog is not a trained service dog either. I don't whether she senses something or maybe my friend smells different when her sugar is lower. Pets really are such wonderful companions.