And my boss knows the contents of the trunk of my car how?
Precisely. How would they know, unless you told them.
here in Michigan it would onl;y be a violation of company policy, not against the law.
Well, many companies have a written policy that says they can search your car if it's in the lot and if you refise they can fire you. So there probably wouldn't be a problem unless they decided to do a search, which companies have done, but then you're screwed if you want to keep your job, unless the law protects you.