A security guard at the British embassy in Berlin who has been arrested on suspicion of passing state secrets to Russia lived in a two-room flat on a tidy estate in the city of Potsdam.

David Smith was arrested at his home on Tuesday and the contents of his apartment are likely to become a focus of the ongoing inquiry into whether he sold documents to “a representative of a Russian intelligence service” in exchange for cash.

On Thursday, Smith’s neighbours had little to say about the 57-year-old British national – a woman sitting in her garden terrace reading, a couple of doors away from his flat, only said he had “kept himself to himself”.

German police left his apartment unprotected, meaning members of the public could easily peer inside through half-drawn shutters.

Whether anything inside has any bearing on the investigation will be assessed by detectives, who will also want to look at the extent of the vetting process that led to him getting a job in the embassy, where he started work late last year.

Photographs published by numerous media organisations on Thursday showed Russian flags inside his flat and military memorabilia from east Ukraine. Other items included a mug bearing the flag of Novorossiya, a name adopted by the separatist republics of Donetsk and Luhansk.

Clearly visible was a framed insignia of Ukraine’s Berkut special police unit, which fought back protesters during the 2014 EuroMaidan protests and were lionised by opponents of the new Kyiv government.

A bookshelf held a partially obscured insignia of the so-called Somalia Battalion, a Russian-backed separatist military unit that fought against the Kyiv government and participated in the battle for Donetsk airport in 2014.

Several of its fighters, who used the battle names Givi and Motorola, were regularly featured on Russian state media.

A sailor’s cap seen on a shelf is part of the uniform of the Black Sea Fleet, which is based in Sevastopol. The city, along with the rest of the Crimean peninsula, was annexed by Russia from Ukraine in 2014.

The items appear to show an interest in a particular period of history – and whether any of it has any relevance to the investigation will doubtless form part of the inquiry led by the German authorities.

There was certainly little about Smith’s flat – for which the current market value is about EUR700-EUR800 (GBP595-GBP680) rent a month – that made it stand out from the others on the estate; from the rubber plants in the window to the neat coloured knives hanging next to a kettle in the kitchenette, which was separated from the living room by a partition wall.

A PlayStation, cuddly toys including a bear in a sombrero and a stuffed doberman, and some artificial floral arrangements were also visible through the window.

Contrary to some earlier reports, Smith worked in Berlin not as a private contractor but as a “local hire” who was directly employed by the British embassy.

While the embassy outsources some security work to Securitas and building management to CBRE, both companies have categorically denied that Smith is or was on their payroll. “The arrested person is not known to Securitas,” a spokesperson for the company told the Guardian.

Local embassy hires usually undergo a shorter vetting process than UK-based civil servants or top-level diplomats representing the country abroad.

Smith was arrested “on suspicion of acting on behalf of a foreign intelligence agency”. The relevant part of the German criminal code, section 99 paragraph 1, applies mainly to espionage against Germany but can be expanded to cover allied states in conjunction with the country’s Nato Troop Protection Act, meaning the suspect could end up being tried in a German court.

German law criminalises “communicating or supplying facts, objects or knowledge” to an intelligence service of a foreign power and is thus more broadly phrased than the British Official Secrets Act, which specifically makes it illegal to pass on a sketch, plan, model, note or secret official password or code word.

Under German law, Smith can be held in pre-trial detention while investigations into his actions are ongoing, and before he has been fully charged.