The private investigation act in practice: invalidity of observations confirmed

An employee, who was also an employee representative and member of the trade union delegation, was suspected of fraudulent mileage records and concealing secondary activities. The employer wanted to verify how the employee spent his time and hired a private investigator, who carried out observations over a period of three months. 

The final report of these observations formed the basis for the intention to dismiss the protected employee for serious cause based on repeated violation of instructions and unlawful enrichment through mileage allowances.

The labour tribunal ruled in the first instance that there was no urgent cause, while the labour court did recognise the presence of an urgent cause.

Both the labour tribunal and the labour court ruled that the private investigator's report was invalid because the maximum observation period had been exceeded. 

According to Article 90 PIA, the duration of the observation per assignment or consecutive assignments with the same purpose must be limited to less than four consecutive or non-consecutive days (ninety-six hours), spread over a month.
 
According to the labour court, the duration of the observation is irrelevant: as soon as an observation took place on a given day, it counts as one day. In any case, non-consecutive observations are limited to less than four days per month and may not be spread over several months. In this case, the limits were exceeded because observations were carried out on more than three non-consecutive days spread over a period longer than one month. 

The court therefore excluded the entire report as evidence of the facts invoked as serious cause. This is not new in itself: the Court of Cassation had previously ruled, albeit not in the context of the PIA, that if the maximum duration of observations is exceeded, the entire observation becomes irregular. Even the first three days of observation may not be taken into account.

Despite the nullity of the observation report, the labour court – unlike the labour tribunal – found that there was still sufficient evidence for serious cause based on evidence other than the report, namely the IT records and email overviews. The employee had entered fictitious addresses, which indicated fictitious movements and repeated violations of instructions.

The PIA provides for a penalty of nullity for non-compliance with specific legal provisions. Only evidence that contravenes these provisions on pain of nullity is excluded in advance.

In other cases (legal provisions not subject to nullity), the Antigoon doctrine applies: evidence may be used unless its reliability has been compromised by illegality or its use would violate the right to a fair trial. This principle was confirmed by the Court of Cassation on 24 November 2025. In this case, the Court of Cassation ruled that a judge must always assess whether the evidence (a private detective's report) is reliable and/or compromises the right to a fair trial.

Employers should strictly comply with the PIA when they call upon private investigators. Violations of observation rules, for example, can lead to the invalidity of evidence and significant financial risks in labour law disputes.

If you have any questions about this law and how to make your company PIA-proof, please contact Sarah Witvrouw or Stijn Lamberigts.

Please also read our previous e-zines in which we discuss the Private Investigation Act in more detail: Fraud Awareness Week.