Making a secret profit is the breach of loyalty that Marshall can be said to have committed here.
This is the term that is used to refer to the fact that in a business arrangement one of the parties has done something that would not be beneficial to the other party as they seek to act or carry out the business that would favor them.
The breach that can be seen here is from the fact that the business was conducted without the agency that Marshall is working for. He acted outside of the Company's knowledge.
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