Once I asked the question and a friend told me that "when" and "where", as the clause markers, are optional in noun clauses but necessary in adverb clauses. However, I can find many examples that the use of the markers is necessary in noun clauses as well.
For example, "Please tell me where Dr. Hopkins office is.

The difference is in the grammatical part the clause plays in the sentence. In your example, the clause 'where Dr Hopkins's office is' (or Hopkin's) plays the part of the object of the verb 'tell'. In the sentence 'I left the cart where Dr Hopkins's office is' the same phrase plays the part of an adverb modifying the verb.

It is important to remember that while 'where' and 'when' would be adverbs if the clause were rewritten as an independent and complete sentence, that role does not determine the nature of the clause.