This is incorrect. Whenever the vehicle is in the shop for any work it is the duty of the technician to inspect the vehicle for any defects or damage. If somthing is found that is covered under warranty they get an ARA(Additional Repair Authorization) slip signed by their shop foreman and the repair gets added to the ticket.
I am an ASE master certified dealer technician BTW.
It honestly depends on the brand. Hyundai/Kia is adamant that technicians sell NO warranty work; actually had a Hyundai trainer at their regional training center tell me that. In a Lincoln dealership I worked at as a technician I was told to disregard (get this) leaking shocks on a Navigator because it was under warranty and the customer had to complain about it for us to be able to fix it. When I was doing parts for a Cadillac dealership, on the other hand, the techs sold warranty work regularly. Don't get me started on Chrysler and all of their mixed signals.