Not true of every garage. Twice I've collected my car after a service & been told that they'd done warrantee work without even being asked to. One of those was the rear light cluster in my current Leon, the other was a clutch and door lock in an MR2.
There are some good dealers there. Same here, they saw my water ingressed rear light cluster, they told me after the service, can you come back in 2 weeks time? We would like to replace that with a new ordered one, but it will take up to 2 weeks to arrive. Then Irealised, oh yea I was meant to tell them about that...