Something similar happened to us.
I employed a dog-walker for a while when I had a bad back and couldn't walk the dog (couldn't walk for longer than 5 mins on the flat at that time).
Anyway, dog-walker turned up (originally he turned up punctually, but then started turning up later and later which in hindsight should have been a red flag). I was at home with back injury and because I work at home I never had to give him a key thank heavens because I could hand over the dog personally.
He usually took the dog away for approx 45 mins or so and when he came back the dog smelled of cigarette smoke (which when I queried it, he said he couldn't smell). Second red flag!
The dog also still seemed quite frisky and needed to go out fairly soon after his walk (third flag!).
Anyway, my dh's office is not far away (which dog walker didn't know) and it turns out that a colleague who knows us and the dog, spotted the dog walker (several times!) in his car, with our dog, smoking 
and playing games on his phone! 
The dog-walker had taken advantage of the fact that I was incapacitated with my back problem and knew I wouldn't be going outside, so thought he wouldn't get caught!
Later on, even worse, it turned out that a friend and neighbour of ours, who used the same dog-walker, having been told about these events by my dh, as a result checked his security camera, on which he saw the dog-walker treating his Yorkie roughly and yanking it up (dog-walker is tall) by the harness in order to pick it up
One of the worst aspects of it is that I had many conversations with this man on my doorstep, when he constantly mentioned his love of dogs, and the special relationship he had with his own 
.