Private investigators and employees on sick leave: what a company can legally investigate
It's one of the questions we get most from HR departments and labour law firms: "We have an employee on sick leave and there are signs they're doing things incompatible with their condition. Can we hire a private investigator?"
The answer is yes.
Private investigation is a lawful tool for a company to verify facts and conduct related to compliance with employment obligations. Law 5/2014 on Private Security expressly assigns private investigators the task of investigating, on behalf of legitimately entitled third parties, facts and conduct relating to the workplace.
The investigator doesn't decide whether a temporary incapacity is medically justified, nor do they replace the physician. They document objective conduct that can later be assessed by the lawyer, by a medical expert when necessary, and by the court.
No prior "well-founded suspicion" required
Supreme Court ruling STS 551/2023, of 12 September, clarified that the lawfulness of surveillance doesn't depend on there being pre-existing well-founded suspicions or a specific number of indications.
That makes sense. If the company could already prove the breach, it probably wouldn't need to investigate.
What the law does require is a legitimate interest and execution that follows reasonableness, necessity, suitability, and proportionality. The absence of prior proof of fraud doesn't make the surveillance unlawful, but it doesn't authorize indiscriminate monitoring either.
There must be a specific employment-related purpose: checking a parallel professional activity, documenting conduct apparently incompatible with an incapacity, verifying a breach of a non-compete obligation, or establishing a possible breach of contractual good faith.
The investigator documents facts; the judge draws conclusions
A person on sick leave isn't required to stay motionless. They can walk, leave the house, travel, or maintain a social life, and depending on the condition, some of that conduct may be compatible with recovery.
That's why a professional investigation doesn't consist of following someone hoping to photograph them "doing something."
The goal must be to document conduct that is objectively relevant to the existing limitations.
A picture of an employee having coffee tells you little. A sequence in which they lift loads, make prolonged physical efforts, or carry out a different professional activity for hours can have an entirely different weight.
At Método 3 we start from one rule: the report describes conduct; it doesn't diagnose illness.
An investigator can write: "At 11:23 a.m. they lift a box with both arms, carry it for twenty metres, and place it in the vehicle." They shouldn't replace that description with "carries out an activity incompatible with their injury."
Where an investigator can investigate
Generally speaking, an investigator may observe and document conduct that takes place in public places or places open to the public, from legitimate vantage points: travel, activity visible in establishments open to the public, loading and unloading, driving, certain sports activities, or lawfully observable gatherings.
The essential rule is simple: the investigator observes what an outside person can legitimately perceive; they do not intrude into reserved spaces.
STS 380/2023, of 25 May, is particularly instructive. An investigator photographed an employee on sick leave doing strenuous work in the garden of their home. The Supreme Court held that this space belonged to the protected sphere of the home or, at the very least, was one of the "reserved places" under the Private Security Act.
The professional conclusion isn't that employment-related surveillance is problematic. It's the opposite: the investigator's value lies in knowing where legitimate observation ends and where an intrusion that shouldn't happen begins.
The expectation of privacy matters
Privacy doesn't work as a binary line between "inside the home" and "outside the home."
Gardens, patios, terraces, private areas, or interior spaces accidentally visible require analysing the specific space, its layout, its relationship to the home, the barriers in place, its degree of exposure, and, above all, the means used to observe it.
Naturally perceiving conduct from a lawful position is not the same as using technology to overcome a physical or visual barrier.
Technology, yes — but with judgment
STS 278/2021, of 10 May, found unlawful a GPS-based surveillance operation that made it possible to know the subject's location permanently and indiscriminately for months.
The difference with traditional surveillance is clear. An investigator stops observing when the person enters a private space and selects only what's relevant to the assignment. Permanent technological monitoring can record a person's entire life.
More technology doesn't always mean better evidence.
Proportionality: the safeguard that protects the report
A proper investigation must be suitable for verifying what it sets out to investigate; necessary, without using more invasive means than required; and proportionate, with an intensity and duration matching the purpose pursued.
These principles aren't an ethical recommendation. Law 5/2014 expressly incorporates them.
Surveillance over a few specific days, during time slots related to the conduct under investigation and with a defined objective, is far more defensible than indiscriminate monitoring.
The report that serves the labour lawyer
An effective employment investigation needs objective facts: date, time, place, duration, and description of the conduct.
It needs complete sequences, not isolated images without context.
It needs targeted surveillance: the best reports don't contain hours of irrelevant observation, but the moments that answer the question that prompted the assignment.
It needs integrity of the recordings and preservation of the originals.
And it needs ratification by whoever observed.
One of the great advantages of an investigator's report is that its author can appear before the court, explain what they saw, and answer questions from both parties.
Investigator and lawyer: the sooner they work together, the better
The investigator investigates facts. The labour lawyer needs legally useful facts. Both should align from the start.
If the end goal may be a dismissal letter, the facts documented need to be specific enough to be included in it: dates, times, conduct, duration, repetition, and circumstances.
Private investigation works especially well when it isn't used as a last-minute resource, but as part of an evidentiary strategy.
Conclusion: strong evidence is clean evidence
There's a very human temptation: "Since we're already following them, let's try to get everything."
That's exactly the temptation to avoid.
The best investigation isn't the one that learns the most about the employee. It's the one that obtains only what it needed to obtain, by lawful means, for the time necessary, and in a way that can later be defended before a judge.
Three genuinely relevant pieces of conduct, observed from a legitimate position, thoroughly documented and reliably ratified, can be worth far more than fifty pages about a person's private life.
In employment investigations, the strongest evidence isn't the most invasive.
It's the most professional.
Truth has a method.