What a private investigator CANNOT do: the catalogue of evidence that can ruin a winning case
In the first entry of this series, we explained why the investigator's report has full standing at trial when it is properly constructed. Today we turn to the other side of the coin: the actions that can turn an investigation into a procedural time bomb.
Because poorly obtained evidence is not merely evidence that becomes weaker. If it infringes fundamental rights, it can be excluded and, in certain circumstances, also compromise other evidence connected to it. It can also give rise to liability for the investigator and, depending on the case, for whoever commissioned the operation.
First distinction: irregular is not the same as unlawful
Forensic practice all too often confuses two distinct categories.
An unlawful piece of evidence is one obtained in breach of fundamental rights. In such cases, Article 11.1 LOPJ comes into play, and, in labour proceedings, Article 90.2 LRJS.
An irregular piece of evidence may have been obtained in breach of an ordinary legal rule without there necessarily being an infringement of fundamental rights. The procedural consequence is not automatically the same, nor can one always speak of constitutional nullity.
The professional lesson is simple: not every defect destroys a case, but every defect opens a flank. And when the operation strays into the territory of fundamental rights, the risk changes scale.
1. Homes and reserved places
Article 48 of Law 5/2014 establishes an essential boundary: the investigator cannot investigate private life as it unfolds inside homes or other reserved places.
The STS 380/2023, of 25 May, is a good example. The Supreme Court held that photographs taken of an employee in the garden adjoining their home were unlawful, given the garden's integration into the domestic sphere and the legitimate expectation of privacy.
The modern rule is no longer simply "if it can be seen from the street, it can be photographed." One must analyse the space, its relationship to the home, its degree of exposure, and the means used to observe it.
2. Intercepting other people's communications
An investigator cannot intercept conversations or communications between third parties in which they do not take part.
Article 18.3 of the Constitution protects the secrecy of communications, and certain acts of interception or unauthorised access can give rise, in addition to evidentiary unlawfulness, to criminal liability under the Criminal Code.
A different matter is a recording made by one of the parties to the conversation themselves. The STC 114/1984 established that someone who records a conversation in which they take part does not, for that reason alone, infringe the secrecy of communications, although its content, purpose, and possible dissemination may raise other privacy concerns.
3. Making the private investigation of publicly prosecutable offences the object of the assignment
The investigator does not stand in for the Judicial Police.
When the assignment specifically has the investigation of an offence as its object, Law 5/2014 confines that activity to offences prosecutable only at the request of a party and on the instructions of those legitimately entitled to bring it.
This does not mean an investigator cannot document lawful economic, employment-related, or commercial facts within a legitimate assignment, and that those facts end up revealing indications of a publicly prosecutable offence. At that point, cooperation with the competent authorities is called for.
The boundary is clear: documenting facts within a legitimate private investigation is not the same as taking on, privately, the criminal investigation of a publicly prosecutable offence.
4. Inducing the conduct one later seeks to prove
The investigator can operate undercover. They can use a legitimate pretext and create an ordinary occasion to observe pre-existing conduct.
What they must not do is manufacture the infringement.
The STS 155/2020, of 19 February, declared unlawful a piece of evidence in which a female investigator, under a cover identity, insisted that an interview be held precisely during the employee's working hours and decisively contributed to generating the absence that was later used to dismiss him.
The operative rule is very useful: the investigator can create the occasion; they must not create the conduct.
5. Permanent and indiscriminate geolocation
The STS 278/2021, of 10 May, held that surveillance carried out by means of a GPS device placed on the investigated person's vehicle for months was unlawful.
The technology made it possible to know the person's location permanently and indiscriminately, twenty-four hours a day, without the natural pauses that exist in human surveillance.
A professional licence does not turn any technically available means into a legally admissible one.
6. Disproportionate surveillance
Even when the means used is lawful, its intensity can cease to be so.
Surveillance of limited duration, directed at a specific type of conduct and carried out during reasonable time slots, does not pose the same problem as an exhaustive monitoring of a person's life over weeks or months.
Reasonableness, necessity, suitability, and proportionality are not a slogan. They are legal criteria that must be capable of being explained afterwards before a court.
7. Accessing data through unlawful means
The fact that an investigation is backed by a legitimate interest does not authorise access to just any data.
One cannot obtain other people's passwords, get into private accounts, intercept communications, penetrate computer systems, or access restricted databases without authorisation.
Open sources and legitimately accessible registers can be part of a professional investigation. Covert access to restricted information cannot.
The derivative effect: there is no automatic contamination
For years the expression "fruit of the poisonous tree" has become popular. It should be used with precision.
In Spain, the exclusion of derivative evidence does not operate mechanically for the mere fact that an unlawful original piece of evidence exists. Since the STC 81/1998, constitutional case law has analysed the so-called connection of unlawfulness between the original infringement and the subsequent evidence.
Unlawful evidence can therefore compromise other derivative evidence when there is a legally relevant connection, but it does not automatically render the entire subsequent proceeding void.
That precision does not reduce the risk. It makes it more realistic.
The conclusion of forty years
At Método 3 we frequently turn down assignments and methods. Not because they cannot technically be carried out, but because the client does not need information at any price.
They need evidence they can use.
An investigator's professionalism is measured by what they achieve, but also by what they know they should not attempt to obtain.
In private investigation, the limit is not the enemy of the result.
It is its guarantee.
Truth has a method.