I own a very large ginger Tom cat. He is supposed to be on a diet but comes home licking his lips and smelling of food. And also, often,
cigarette smoke 
I bought a "cat nav" device for him (v small USB thing which attaches to his collar). I have used it three times now and downloaded the data and it shows him going into a house about 8 doors down and not moving for an hour.
AIBU to go and speak to them and ask them to stop feeding him? I wasn't planning on telling them about the GPS device... 
Or would you leave it?
I am aware this post reads like I am a mad woman, btw. I promise I am sane. I just can't bear the patronising vet telling me he will get type 2 diabetes if he doesn't lost weight 