When I had my Revo'd Ibiza Cupra in at a SEAT garage (which has actually closed down now) for some warranty work, one of the "tech's" was told to take it out to try to identify a noise on it. He wheel spun out of the forecourt and bounced off the rev limiter, screaming down the road and back. When he pulled up again, he threw the keys at me from about 10 feet away then strutted off like nothing was wrong.
That is completely unacceptable behaviour in someone elses car. I believe he was let go very shortly after that.....
Like RedRobin says, find a garage you trust, but I'd still make a note of the mileage when you hand it in.