It is determined by that which is written in the document itself.
In other words, this is yet another question you can't answer, because you can't come up with a single case.
Of all the liberal law groups out there that are willing to jump on any case pro bono that involves the cops and has a chance to make it to the Supreme Court, you can't come up with a single one.
In other words, one does not look to whether one government employee investigates the behavior of another government employee to determine if the latter’s actions were constitutional.
One looks to the Constitution itself.
One can also look to Robert Peel’s nine principles for policing to determine whether such conduct is within the limited bounds established by said principles. Who better, after all, to inform the public of the principles to which the police must abide better than the Father of Modern Policing?
1. The basic mission for which the police exist is to prevent crime and disorder.
2. The ability of the police to perform their duties is dependent upon the public approval of police actions.
3. Police must secure the willing co-operation of the public in voluntary observation of the law to be able to secure and maintain the respect of the public.
4. The degree of co-operation of the public that can be secured diminishes proportionately to the necessity of the use of physical force.
5. Police seek and preserve public favour not by catering to public opinion, but by constantly demonstrating absolute impartial service to the law.
6. Police use physical force to the extent necessary to secure observance of the law or to restore order only when the exercise of persuasion, advice, and warning is found to be insufficient.
7. Police, at all times, should maintain a relationship with the public that gives reality to the historic tradition that the police are the public and the public are the police; the police being only members of the public who are paid to give full-time attention to duties which are incumbent upon every citizen in the interests of community welfare and existence.
8. Police should always direct their action strictly towards their functions, and never appear to usurp the powers of the judiciary.
9. The test of police efficiency is the absence of crime and disorder, not the visible evidence of police action in dealing with it.
It seems rather clear that Robert Peel would not be scrambling to defend their actions by trying to chase a rabbit trail or two.
In other words, I have no interest in chasing the rabbit trails which you are so desperate to incite others to follow you upon because you cannot justify the actions of your beloved government employees using either the Constitution or Robert Peel’s own principles.