That's why no one touches my cars unless I'm watching.
I learned this a long time ago when I was in highschool. A friend in the autoshop had his family car, a '73 Charger, brought in to Midas to have all 4 wheel cylinders changed. After the work was done, we brought his car in the shop and none of the wheel cylinders were changed but he was charger for it.
Anther time I dropped off my wife's car to have an inspection sticker done, and after a week or so I realized that they swapped my good lugnuts for crappy ones.
When I used to work for Acura of Boston, one of the techs there used to keep the oil filters on the customer cars, not change thiers, and use the new filters on side jobs that he would charge other people for the new filter he didn't even pay for!
But there are good mechanics out there. You just have to be very careful!