A suspected far-right sympathiser is feared at large in the Metropolitan police, having got away with scrawling a swastika in a secure area of a police station, the Guardian has learned.

The hate crime was not made public by Britain’s largest force at the time and the culprit has not been caught.

The swastika was found in February drawn on an inside wall at Edmonton police station in Enfield, north London, in an area only accessible to officers and staff.

The police watchdog, the Independent Office for Police Conduct (IOPC), said it had not been informed of the incident nor made aware of Scotland Yard’s investigation into it.

The Met said it had launched a thorough investigation into the swastika graffiti, which it had classed as a faith-based hate crime, but could not identify the perpetrator.

The former chief superintendent Victor Olisa, head of diversity at Scotland Yard from 2016-17 and former commander of the borough of Haringey, said: “How can this happen in a part of the station where only officers and staff go, and someone feels confident enough to draw a swastika in a police station.

“People have been saying things are getting worse on diversity and equality in the Met.

“It could be someone with far-right sympathies who is confident to express that in a secure part of a police station.”

Olisa said the incident would be “deeply shocking and upsetting” for minority ethnic officers in the station. “I’m not sure why it is being treated as a faith-based hate crime. It seems to me to be more extreme-rightwing.”

In a statement the Met police said: “On 15 February 2019 graffiti was found on the wall in the lift lobby area of Edmonton police station. The graffiti, in biro, was a swastika symbol.”

The Met said it treated the incident seriously, adding: “The incident was recorded as a faith hate crime and a senior investigating officer at detective chief inspector level was appointed to investigate the offence.”

The Met confirmed the area the swastika was found in was not accessible to the general public and could only be accessed by those who work for the force and have clearance: “A thorough investigation was undertaken but no forensic opportunities were identified. As a result, it was not possible to identify who had drawn the graffiti.”

The Met declined to answer if any employee was spoken to or interviewed under caution, and added: “We take all hate crime seriously and have a zero tolerance approach.”

Iman Atta, the director of Tell Mama, a national project that documents hate crime, said: “To have a swastika, a Nazi symbol, in an area which police officers and staff alone have access to, is worrying.

“While this is an isolated incident, it is deeply concerning. This also raises questions about the institution and whether such incidents are truly isolated.

“The local force should undertake a thorough investigation to reassure communities that there is zero tolerance towards any form of racism within the force.”

The former Met superintendent Leroy Logan said he had to deal with racist graffiti on his locker while serving in the 1980s and the news of the swastika incident left him worried that things were going backwards in the capital’s police force.

He said the potential effect on officers could be significant: “It stuns you, it makes you fearful and angry at the same time. You think: ‘These are the people I work with, and my life may be in their hands.’”

Logan said the incident would also undermine community confidence “These people [extremists] have infiltrated the police culture and can impact how a diverse population is served.”

Police rules ban officers or staff supporting extreme rightwing groups.

A spokesperson for the IOPC said they had not been informed of the incident: “Police forces must refer the most serious incidents to us – whether someone has made a complaint or not. Police forces can also refer incidents to us if they have concerns, for instance about the conduct of their officers or staff.”

Olisa said: “I’m surprised it was not referred. You refer something to the IOPC where there is likely to be reputational damage, damage to public confidence or potential operational culpability.

“I am really surprised the Met has not dealt with it as robustly as I would have expected for an organisation that takes hate crime and integrity seriously.”