Loosing parts while servicing bikes

My bike went for a very minor service on Friday and when I got it back the safety cap bove the shick absorber was missing exposing the nut that *****s on and off. When i asked where the safety cap was they said there was not one but didn't even go into the back to check. They could not even careless and what surprises me is how little they know about bikes. I want them to take responsibility and either find my missing safety cap or replace it. How can you leave a nut exposed?? Please sort out the lack of knowledge and poor service given by this branch.