There is some merit in this.
I am not for one moment suggesting the poster's garage is "playing games" but some do and manufacturers are also warey.
I recall a discussion with a technician for a particular franchise saying that they had battery failures and the new batteries also failed as soon as they came off the truck or some short time later. This built a level of questions and/or distrust with the manufacturer thinking the authorised dealer was playing a warranty game where they get all the labour costs for replacing an item.
There is no doubt that warranty fraud claims happen. I know for fact that Fiat Zone Managers used to smash holes in the sides of engine blocks so engines replaced under warranty could not be "re-distributed" for extra revanue.
If you are out of warranty then you have the legal right to have all replaced parts, for which you have paid, to be returned to you. No doubt some bad boys will return a part that was not yours unless you are present at removal or have invisibly marked the part for yourself.
In warranty then you have no rights as you are a 3rd, if not 4th party, in the loop. However IMHO it is always a good practice to approach/treat all warranty service actions as if you had paid for these yourself out of warranty.
Please NOTE! We are Not neccessarily trying to trip up a garage/dealer up but as a car lover driver and road user it behoves you to try and understand all the potential *killer* issues one could encounter in one's vehicle.
