The service advisors and mechanics get paid commission on the repairs so incentive is to find EVERYTHING.
My favorite thing to do: if I'm paying for new parts to be installed, I want the old ones back and I repeat it several times to the service advisor to let him know I'm not joking. Even got a quarter panel back for an FJ Cruiser one time to make sure I got a new part and not bondo.
My wife's daily driver is a Honda CRV (it's what she likes...so...). The first Honda dealer we went to, we agreed on a price, handshake, done. Right? Wrong. We sit down and find $250 clear coat added to the final sale. Wasn't on the sticker. Never part of the discussion. What's this? Salesman said we put a clearcoat on every car to protect the paint. (????) I said this was not even legal to do. It needs to be on the itemized window sticker not to magically appear buried on the bill of sale. I said take it off or no sale. Manager came in and said we can't take it off. (???) We magically went to a competing dealer down the road and bought the exact car without the special clearcoat.
A public service announcement. While I am not an authorized mechanic, I have several friends who are. Here is what you do when faced with a potentially large auto service issue: Ask the mechanic what he would do if it were his car. This is their kryptonite.