Mere ideas are not, as others have said, protected by copyright. However, the police officer in such a situation may have a duty of confidentiality, particularly if s/he is informed that the contents of the phone are confidential.
For example the "Officer's Code of Conduct" of Canton Ohio, says:
Whatever a Police Officer sees, hears or learns of that is of a confidential nature will be kept a secret unless the performance of duty or legal provision requires otherwise. Members of the public have a right to security and privacy, and information obtained about them must not be improperly divulged.
Many police departments have similar codes of conduct or policies. Such a policy might apply in the circumstances described in the question.
In this page on "Confidential information" posted by the TaylorWessing law firm, it is said that:
There are three circumstances in which the disclosure of information gives rise to an obligation of confidence:
- An obligation imposed by contract – for example, non-disclosure agreements.
- An obligation implied because of the circumstances of disclosure – this is determined on the basis of whether a reasonable man standing in the shoes of the recipient of the information would have realised that the information was being given to him in confidence.
- An obligation implied because of the special relationship between the parties concerned – for example, employer/employee relationships.
The best way of ensuring that confidential information is recognised and treated as such is by contract. If no express requirement for confidentiality is imposed, the manner in which the discloser treats the information can be held as indicative of its level of confidentially. Talking about information in public places, for example, would cause the recipient to assume it was not confidential in nature.
An action for breach of confidence is based on the conscience of the recipient who must have agreed or known that the information is confidential. Thus an action for breach of confidence can be brought against the original recipient of the information and any subsequent recipient who becomes aware of the fact that the information is confidential.
An action for breach of confidence may be brought against a person who has used or disclosed, or threatened to use or disclose, confidential information without permission.
A police officer engaged in a search of confidential documents, whether found on a phone, on a computer, or in a filing cabinet, is in a "special relationship" with the sub ject of the search, and the "circumstances of disclosure" are unusual. Use of such information for private, personal gain unrelated to any police function might be held to violate an implied duty of confidentiality, particularly if the officer is informed of the confidential nature of the documents.
If the documents contain or constitute trade secrets, unauthorized use will give grounds for a lawsuit. This would not apply to proper use within a police investigation or criminal case, but it would apply to private use or disclosure by the officer.
in "Release of personal information to
police: your privacy rights" from the Government of Ontario it is said that:
Ontario public sector organizations, such as provincial ministries and agencies, municipalities, schools, and transit systems, are required by
law to protect your personal information and to follow certain rules when collecting, using, and disclosing your personal information.
This document is primarily about when other governnment agencies can release personal information to law enforcement, but it might be taken to imply a similar obligation to respect privacy being imposed on law enforcement officers.
In this page and other linked pages the UK metropolitian police indicate that they are subject to the Data Protection Act 2018 This is closely related to the GDPR. This restricts processing of personal data to lawful purposes. In the absence of the consent of the person searched, I doubt that use of information for personal gain having no connection with a police investigation would constitute a lawful purpose.
I have not found an authoritative source, nor a court case, in which the situation described in the question, nor any closely similar situation, is addressed or decided.