Checked wallpaper decorated with a Matisse poster and a miniature portrait of the Virgin Mary. A black leather sofa. A single bed with electric-blue sheets. An Ikea rice-paper lampshade. A Star Wars novel and a catalogue of African string instrumentson the floor.

At first, the underground workers who stumbled upon this scene in a disused U-Bahn tunnel in Berlin’s Reinickendorf district in mid-January assumed they had encountered an abandoned film set. But checks showed that no film recordings had been applied for in the area. The wallpaper, furniture and books were deemed a fire risk and were swiftly removed.

A month later, well-lit photographs of the bedroom were emailed to the biggest Berlin newspapers by someone claiming to be an anonymous rail worker. Berlin’s transport body, BVG, denied that its staff had anything to do with it.

Then, a second domestic tableau appeared in early March, this time in an underground passage at Yorckstrasse station: a front garden complete with fence, garden gnome, flower bed and a cat staring through a window.

Since then, Berliners have been speculating about which artist could be behind the underground rooms and what it could all mean. An advert posted (and since removed) on Airbnb alongside a photo of the bedroom points towards a critique of the German capital’s rapid gentrification. “Central location, close to the U9 line,” the ad reads. “Loft character, retro-chic. The apartment was completely modernised in 2016 and is relatively quiet (as long as you wear earplugs). Perfect for commuters and rough sleepers.”

Among the graffitied concrete walls and industrial stairs, the bedroom looks pretty cosy.

Others have suggested that the clues may lie in the books. The battle theme of the Star Wars novel and the book on African string instruments could be a reference to the refugee crisis, pondered architecture critic Nikolaus Bernau in Berliner Zeitung newspaper, pointing out that the Berlin senate had sheltered some refugees in its windowless congress centre. Another writer noted the catalogue of instruments was published in 1984 – clearly the installation was a commentary on the surveillance state and the end of privacy.

That the artists behind the rooms have stayed anonymous is hardly surprising. BVG says the creators must have gained illegal access to the tunnel – the least they could expect would be a fine for trespassing. “We cannot condone these works,” says Petra Reetz, a spokeswoman for the company. “But the attention to detail that’s gone into these rooms, it’s quite original – I have to admit that.”