It is often not easy to gather evidence about a behaviour or a fact. The known presence of a private investigator can change the behavior of the swindler and alter the reality of things. Sometimes a ruse -or some kind of artifice- is the only way to get information, in order to bring the truth to light. A ruse is, for example, assuming another capacity (character), telling a fictitious story in order to obtain the evidence sought from the person concerned. A ruse can be used if there is no other efficient way to obtain equally reliable evidence.
For instance, an entrepreneur who suspects that his stolen goods are being sold online and therefore engages a private investigation agency. In such a case, the private detective tries to make online contact with the seller and to book an appointment to find out whether or not the serial numbers correspond to the stolen goods. If the investigator is 100% sure that the purchase is a transaction prohibited by law (e.g. narcotics), then the test purchase may not take place under any circumstances.
As an investigator we can, amongst other things, assume the capacity (read: character) of client or purchaser, as long as we do not assume a protected title. Thus, a private investigator cannot assume the capacity of, for example, architect, notary, doctor, etc. unless the investigator meets the legal requirements. Assuming the capacity of a civil servant is also prohibited by law.
Even if we impersonate someone else during a mystery guest assignment, we still objectively determine what person concerned says and does. It is absolutely inappropriate to ask suggestive questions. The use of a ruse in combination with putting pressure on the person involved is legally unacceptable.