Anything is possible but…..
Personally. I have never seen or heard of a rock or something loosening a caliper bolt in almost 50 years of working on cars.
I have heard of rocks jammed to the head of a caliper bolt but never loosening one.
I think the dealership is trying to cover their butt.
Just for giggles, do a search on google, yahoo and any other search engines and look up rock loosening caliper bolt.
Print out the results and show the person who told you that foolishness.